All of the passengers were Thais 60 years old or older.
Authorities were still trying to raise the bus yesterday.
The bus, heading from Phrae to Bangkok, ran off a winding downhill road on Doi Khao Pueng mountain into the ravine at Haiha village in Tambon Ban Dan Nakham at about 10am.
“The route is particularly steep and there have been various accidents at this spot. But this is the first time the casualties are this many,” Uttaradit police chief Pol Colonel Ditsayadej Patcharapuwadol said.
An initial investigation concluded the accident was probably caused by the driver’s lack of familiarity with the route. The driver was being treated at a local hospital and in police custody yesterday.
The death toll could increase, as some passengers remained stuck in the bus as of press time yesterday, although most had been taken to a nearby hospital.
Police said rescue operations were made difficult because of the ravine’s depth, adding that an army crane would be used to raise the bus.