The accident occurred at 5:00 am (0200 GMT), the TRT public televisionchannel said. It was travelling from Eskisehir, a city west of Ankara.
Ankara region governor Vasip Sahin said preliminary information suggested thedriver had fallen asleep as there were no indications he had tried to brake.
An investigation has been launched.
Bus accidents are fairly common in Turkey.
In May, 10 people died and 40 were injured when a bus and two cars and atruck collided on a highway near the southern city of Mersin, reports BSS.