Pol Lt Sakchai Chanthana, the duty officer of Bang Khanak police station, said the accident occurred about 4.50am. The van, carrying 16 passengers, departed from Prachin Buri about 4am and was heading for Victory Monument in Bangkok.
The truck, which belongs to Chokwongthong Construction Co, was parked on the side of the road. Its tail lights were not turned on, he said.
Nine passengers, six men and three women, were killed in the crash. The six other passengers and the driver were seriously injured and admitted to Bang Nam Priew Hospital.
Four of the dead passengers have been identified - Pongsak Luadthai, 21, of Prachin Buri, a student at the Merchant Marine Training Centre in Min Buri; Adisorn Penyoo, an employee of Thai Nakhon Pattana Co; Nathapol Baopoe, 31, of Udon Thani; and Chatinai Choeyprasert.
Only one of the three female passengers killed in the accident was identified. She was Sarinla Luesanan, 43, of Bangkok's Chomthong district.
Pol Lt Sakchai said Viroj Wongkot, 30, of Nakhon Pathom, the driver of the truck, was arrested. The van driver, Anusorn Srichai, was among the seriously injured.