The blast ripped through the shafts of the mine in Shaanxi province on Wednesday evening, when 36 miners were at work.
Seven escaped and the others were trapped, the official Xinhua agency said.
China's mining industry is the world's most dangerous.
A total of 3,786 coal miners died in gas blasts, floodings and other accidents last year as companies, often flouting safety regulations, rush to feed demand from a booming economy.
This mine was a state-owned local operation, with annual production capacity of 330,000 tones, the report said.