The bus from Khon Kaen was heading to Chiang Rai province and crashed into the guard rail of a bridge which has the tallest supporting pillars in Thailand, and fell down, police said.
Police said the bus was carrying 40 people, 27 bodies were found in the bus and many more outside. Five severely injured were rushed to two hospitals and one of them died at a hospital.
The bus from Khon Kaen was heading to Chiang Rai province.
The ravine is about 50 metres deep and rescue workers are still combing the site. The mishap occurred as millions of Thai motorists are expected to travel during the New Year holidays.
Earlier in October, at least 20 people were killed when a tour bus carrying elderly Buddhist devotees plunged into a ravine in northeast 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.