The official Xinhua news agency, quoting safety board officials, said the two carriages with a total of 34 miners aboard slipped and overturned when a steel wire connected to them snapped about 260 feet from the entrance of the colliery pit in Baiyin city in Gansu province in northwest China.
All the dead miners have been taken out of the pit, along with another 14 survivors, while rescue operations continued for the seven trapped miners, Xinhua said.
The coal mine began operation in 2003 with an annual output capacity of 90,000 tons.
This is the latest deadly mine disaster to strike China, where a rash of such accidents in recent weeks has claimed dozens of lives.
At least 245 miners have died in 62 mine accidents so far this year, official media have reported.
In one of the most disastrous incidents this year, a mine explosion in August in southwest Sichuan province killed at least 40 miners.