Fifteen people died at the scene, including the driver, Chavalit Thavornkham, 24, the Bangkok Post reported.
Three others died at Buri Ram hosital. Other truck passengers were injured.
Investigators said the truck ran off a road and struck a tree.
Police Col. Suthee Setthawong said many of the passengers had no identification on them. He said he believed many of them came from Laos.
Investigators said they suspect Thavornkham fell asleep and lost control of the truck. They found a methamphetamine tablet in his clothing and a bottle of alcohol in the driver's side door pocket.