A passenger coach run by the Shanghe County Long-distance Transportation Company of Jinan City hit a sedan then broke through median barriers and collided with another coach travelling in the opposite lane, Wang Jiguo, chief of the Zibo Traffic Police Brigade said.
The second coach, run by the Jinan-based Lushun Tourist Automobile Company, rolled over into a trench alongside the highway, which links Qinghai to Yinchuan.
Six people died on the spot.
Seven died in hospital while nine others were seriously injured, state-run Xinhua news agency reported.