The explosion occurred at the Weijiadi coal mine in the north-western province of Gansu on Tuesday afternoon when 39 miners were working underground, Xinhua news agency reported.
Four escaped unhurt, it said, adding that rescue operations were underway.
The cause of the explosion is still unknown, Xinhua said.
China's coal mines are regarded as the most dangerous in the world with almost 6,000 workers dying in the industry last year -- a rate of about 16 fatalities each day -- according to official figures.
Labour rights groups say the real number of mining deaths could be as high as 20,000 each year, with the official tally lower because local government officials and mine owners often cover up accidents.

Copyright AFP (Agence France-Presse), 2006