Police said the accident happened on a mountainous road in Ban Nong Phak Nam village in Tambon Mae Yao at 5.30pm.
The truck’s driver, Kham-ai Phamung, 50, was killed along with a worker, Sawat Suphajai, 54. Both were residents of Tambon Mae Yao.
The two men were apparently delivering construction materials to a buyer in Ban Huay Khom village, but on a steep slope the truck lost power and began sliding backwards until it left the road, plunged 15 metres into the ravine and flipped.
It took rescuers about three hours to extricate the bodies from the battered truck before sending them for autopsy at the Chiang Rai Prachanukroah Hospital.