Mizuho Financial Group's net profit doubled in the October-December quarter, helped by lower credit costs, and it kept its full-year profit outlook unchanged.

Profit growth at Japan's second-largest bank by assets slowed from the preceding two quarters, however, in the absence of hefty bond trading gains, while core lending activities remained sluggish amid the fragile economic recovery.

Net profit came to 80.3 billion yen ($978.1 million) in the third quarter, up from 38.5 billion yen a year earlier, according to Reuters' calculations from nine-month results.

That compared with forecasts of 100 billion yen by Citigroup Global Markets and 87 billion yen by Credit Suisse.

For the first six months of the current financial year, Mizuho and its rivals saw their earnings boosted by a sharp rise in prices of U.S. Treasury and Japanese government bonds, although bond prices started falling late last year.

For the year ending in March, the bank retained its net profit forecast of 500 billion yen, up from 239.4 billion yen a year earlier and in line with an average estimate of 507.3 billion yen in a poll of 14 analysts by Thomson Reuters I/B/E/S.

Last week, No. 3 Japanese bank Sumitomo Mitsui Financial Group posted a 21 percent fall in net profit on poor demand for loans and bond trading losses.

Shares of Mizuho lost 8 percent last year, underperforming a 3 percent fall in the benchmark Nikkei average.

($1=82.10 Yen)

(Reporting by Taiga Uranaka; Editing by Muralikumar Anantharaman and Edmund Klamann)