The double-decker bus, belonged to Phetprasert Tour Co, was on the way from Trat to Mae Sot with about 45 people on board, including some Myanmar passengers.
As the bus, driven by a driver identified only as Noi, arrived near Kilometre marker 413 of highway in Khlong Khlung district of Kamphaeng Phet, it collided with an oncoming 18-wheel truck with full load of sugar which was from Nakhon Sawan.
The head-on collision killed both drivers of the bus and truck and two other passengers, who include a Myanmar man and a woman.
All the injured were later rushed to Khlong Khlung district hospital. one of them died on arrival at the hospital.
Initial investigation by Khlong Khlung police revealed that the bus was on the lane of the truck while the bus driver tried to overtake another vehicle before reaching a road construction site where the highway lanes were reduced to two.
The collision threw the bus off the highway and overturned, spilling some passengers out.