* Q3 net profit up 51 percent to 3.05 bln yuan vs 2.84 bln yuan analyst estimate
* Average revenue per user up 0.6 percent to 48.6 yuan
* Five-quarter run of year-on-year profit gain is longest ever (Adds subscriber growth and average revenue per user, competition and share price)
By Paul Carsten
BEIJING, Oct 24 (Reuters) - China Unicom (Hong Kong) Ltd , China's second-biggest mobile carrier by subscribers, cemented its longest run of year-on-year quarterly profit gain as it signed up mobile phone users at a quicker pace than that of its competitors.
China Unicom has been catching up with bigger rival China Mobile Ltd. It raised marketing costs by 26 percent over the past nine months and increased its mobile user numbers in July-September by 4 percent - twice the rate of China Mobile.
Net profit rose for the fifth straight quarter in July-September to 3.05 billion yuan ($501.36 million) from 2.023 billion yuan a year earlier, according to calculations by Reuters based on nine-month earnings results released after market close on Thursday.
The result compares with the 2.84 billion yuan mean estimate of four analysts polled by Reuters.
Shares in the company ended the day up 1.7 percent compared with a 0.7 percent fall in the Hang Seng Index.
China Unicom shares have fared better than those of competitors, losing 1.6 percent year-to-date compared with 11.5 percent for China Mobile and 6 percent for third-ranked China Telecom Corp.
China Unicom had a 22 percent share of China's mobile network market by users as of August, according to the most recent data provided by the three carriers. It had 40 percent of customers on its more profitable third-generation network, compared with 21 percent for China Mobile and 52 percent for China Telecom.
The company's average revenue per user (ARPU) was low in January-September at 48.6 yuan, with that of 3G subscribers being almost double that of 2G customers.
The ARPU of China Mobile, the world's biggest mobile carrier by number of users, was 66 yuan in the same period, and that of third-ranked China Telecom was 54.3 yuan for January-June. ($1 = 6.0835 Chinese yuan) (Additional reporting by Clement Tan in HONG KONG; Editing by Christopher Cushing)