Twenty Myanmar migrant workers died on Friday. A bus taking them from a Thai border town to Bangkok.
It was not immediately clear how the blaze started on the bus.
But television footage showed a bus destroyed by fire which trapped many passengers inside.
The death toll is 20, three people injured, Pollawat Sapsongsuk of the Tak Disaster Prevention and Mitigation centre said.
Kittisak Boonchan, a rescue worker in Tak province, told reporters there were 47 people on the bus.
The accident took place at 1:25 am in the northwestern province of Tak which borders Myanmar, a source of much of Thailand’s huge migrant labour force.
Migrant workers often fall victim to safety lapses and exploitation in Thailand.
The kingdom has a poor road safety record on crashes with around 24,000 people perishing on its roads each year, as reported by the World Health Organisation.
Last week 18 people died when a bus veered of the road and smashed into a tree in the northeast of the country.
The driver of that bus later confessed to being on drugs in the mean time of the crash.
