The accident occurred on a busy highway in Bahawalpur.
The van, which was going to Ahmedpur Sharqia from Bahawalpur, collided head-on with the truck.
Twelve people were killed instantly and 12 others died in hospital, the officials said.
Seven women and three children were among the dead.
Several people were injured and some of them were in a critical condition, the officials said.
Rescue teams had to cut through the mangled metal of the van to remove bodies and the injured.
