The victims were returning from a religious ceremony when the vehicle left the road, Police Colonel Somdet Tosporn told AFP by telephone from Lampang province.
"The bus fell into a ravine about 10 metres deep," he said. "I can't confirm the cause but the road is curvy."
No foreigners were believed to be on board.
Deadly road accidents are common in Thailand.
A recent report by the World Health Organisation said the country saw some 38.1 road deaths per 100,000 of population, compared to an average of 18.5 in Southeast Asia as a whole.
In July at least 19 people were killed when a double-decker coach collided with a truck in central Thailand and burst into flames.
In April at least five people were killed, including a seven-month-old baby, and 53 were injured when a Thai tour bus plummeted off a hillside in the northern province of Phitsanulok after its brakes failed.