At least 30 people died and 20 others were injured late Monday when a bus careered off a hillside road and plunged into a ravine in northern Tak province, police said.
Rescue workers tried to assist passengers trapped in the bus after a plunge into a ravine in Tak.
The accident happened around 8.40 pm. Four buses were taking local government workers and elderly people to Ubon Ratchthani and to Laos for a field trip.
"The brakes failed as the bus came downhill on a hilly road and it crashed through the concrete barrier and fell into a 150 metre-deep ravine," Pol Capt Sittichai Panyasong of Mae Tho district in Tak province said.
The victims were mainly believed to be local government officials, but a child was also among the injured, he said.
Tak provincial governor Suriya Prasatbunditya said motorists who saw the acident told police the bus driver was trying to pass cars on a winding downhill road when the vehicle skidded off the edge and flipped several times as it tumbled about 30 metres into the valley before crashing into a tree.
The confirmed toll was reported as 30 dead on Tuesday morning, with some reports putting the figure at 32.
Thailand's roads are among the most dangerous in the world.
A recent report by the World Health Organization said Thailand recorded 38.1 road deaths per 100,000 people in 2010 -- behind only the Dominican Republic in the Caribbean and the South Pacific island of Niue.
That compares with an average of 18.5 per 100,000 in Southeast Asia as a whole.
At least 13 school children died last month when their bus collided with a lorry on field trip to the beach.
The students, aged 10 to 14 years, were heading to Pattaya from the northeastern province of Nakhon Ratchasima.
Officials say roughly 60% of traffic accidents in Thailand are caused by human error, with poor road and vehicle conditions posing additional hazards.
Alcohol also plays a significant role, particularly during national holidays, when millions of revellers return to their homes across the country.
Bus operators are required to provide seat belts but passengers are not legally obliged to use them.
In December dozens of people were killed when a bus carrying New Year travellers plunged off one of Thailand's highest bridges in the Northeast.
At least 20 people were killed in October when a tour bus carrying elderly Buddhist devotees fell into a ravine, also in the Northeast.