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SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 6-K

Report of Foreign Private Issuer

Pursuant to Rule 13a-16 or 15d-16 of

the Securities Exchange Act of 1934

For the Month of March 2015

Commission File Number 1-15028

China Unicom (Hong Kong) Limited

(Exact Name of Registrant as Specified in Its Charter)

75/F, The Center,

99 Queen’s Road Central, Hong Kong

(Address of principal executive offices)

(Indicate by check mark whether the registrant files or will file annual reports under cover of Form 20-F or Form 40-F.)

Form 20-F  x            Form 40-F  ¨

(Indicate by check mark if the registrant is submitting the Form 6-K on paper as permitted by Regulation S-T Rule 101(b)(1):     .)

(Indicate by check mark if the registrant is submitting the Form 6-K on paper as permitted by Regulation S-T Rule 101(b)(7):     .)

(Indicate by check mark whether the registrant by furnishing the information contained in this Form is also thereby furnishing the information to the Commission pursuant to Rule 12g3-2(b) under the Securities Exchange Act of 1934.)

Yes  ¨            No   x

(If “Yes” is marked, indicate below the file number assigned to the registrant in connection with Rule 12g3-2(b):82-            .)

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EXHIBITS

FORWARD-LOOKING STATEMENTS

This announcement contains certain forward-looking statements within the meaning of Section 27A of the Securities Act of 1933, as amended, and Section 21E of the Securities Exchange Act of 1934, as amended. Such forward-looking statements may include, without limitation, statements relating to the Company’s competitive position; the Company’s business strategies and plans, including those relating to the Company’s networks, products and services, as well as sales and marketing, in particular, such networks, products and services, sales and marketing in respect of the Company’s 3G and 4G business; the Company’s future business condition, future financial results, cash flows, financing plans and dividends; the future growth of market demand of, and opportunities for, the Company’s new and existing products and services, in particular, 3G and 4G services; and future regulatory and other developments in the PRC telecommunications industry.

The words “anticipate”, “believe”, “could”, “estimate”, “intend”, “may”, “seek”, “will” and similar expressions, as they relate to the Company, are intended to identify certain of these forward-looking statements. The Company does not intend to update any of these forward-looking statements and are under no obligation to do so.

The forward-looking statements contained in this announcement are, by their nature, subject to significant risks and uncertainties. In addition, these forward-looking statements reflect the Company’s current views with respect to future events and are not a guarantee of the Company’s future performance. Actual results may differ materially from those expressed or implied in the forward-looking statements as a result of a number of factors, including, without limitation:

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Please also see the “Risk Factors” section of the Company’s latest Annual Report on Form 20-F, as filed with the U.S. Securities and Exchange Commission.

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SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned, thereunto duly authorized.

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Exhibit 1

Hong Kong Exchanges and Clearing Limited and The Stock Exchange of Hong Kong Limited take no responsibility for the contents of this announcement, make no representation as to its accuracy or completeness and expressly disclaim any liability whatsoever for any loss howsoever arising from or in reliance upon the whole or any part of the contents of this announcement.

CHINA UNICOM (HONG KONG) LIMITED

(Incorporated in Hong Kong with limited liability)

(Stock Code: 0762)

Announcement

LTE/4G Digital Cellular Mobile Service (LTE FDD) Operation Permit

This announcement is made pursuant to Rule 13.09 of the Rules Governing the Listing of Securities on The Stock Exchange of Hong Kong Limited and Part XIVA of the Securities and Futures Ordinance (Cap. 571).

On 27 February 2015, China Unicom (Hong Kong) Limited (the “Company”) was notified by its ultimate parent company, China United Network Communications Group Company Limited (“Unicom Parent”), that Unicom Parent has been granted the license to operate LTE/4G digital cellular mobile service (LTE FDD) by the Ministry of Industry and Information Technology of the People’s Republic of China (“MIIT”) on 27 February 2015. MIIT has also granted approval for Unicom Parent to license China United Network Communications Corporation Limited , a wholly-owned subsidiary of the Company, to operate LTE/4G digital cellular mobile service (LTE FDD) nationwide in China.

The Company will continue to pursue its strategy of “Leading Mobile Broadband and Innovating Operational Integration”, expand its mobile broadband network coverage and increase its network speed, in order to further consolidate and enhance its core competencies.

Hong Kong, 27 February 2015

As of the date of this announcement, the Board comprises: