The truck-trailer loaded with red grapes was on its way to Bangkok while the coach, with 11 passengers, was travelling to Chiang Rai's Chiang Khong district when the accident occurred around 8pm in tambon Huai Lan of Dok Kham Tai district.
Karn Phothong, 49, who was driving a tanker truck behind the coach on Sai Phayao-Chaing Kham road, told police that the 10-wheeler lost control as it sped down a slope, hitting the Sombat Tour coach, which was going up the hill.
The coach plunged into a ravine, quickly followed by the tractor-trailer, which skid for a short distance before going off the road. Mr Karn's vehicle also was damaged slightly.
Sophon Pitaksalee, an official from Phayao Transport office, said four men, including the bus driver, were killed in the accident. One was a monk.
Six bus passengers and the truck driver were rushed to two nearby hospitals.
The police are investigating the actual cause of accident.