The bus, coming from Khon Kaen was enroute to Chiang Rai province, crashed into the guard rail of a bridge in Phetachbun district and fell into the 50-metre deep ravine, police said.
The bus was carrying 40 people, they said, adding 27 bodies were found in the bus and many more outside.
Five critically injured were rushed to two hospitals where one of them died.
Information about the remaining three persons was not immediately available.
The mishap occurred as millions of Thai motorists are expected to travel during the New Year holidays.
According to a recent report by the World Health Organisation, Thailand witnessed some 38.1 road deaths per 100,000 of population, compared to an average of 18.5 in Southeast Asia as a whole.
