The bus was coming from provincial capital Quetta when it collided with the truck in the remote Uthal area, police said.
The truck driver died on the spot.
A police official said that 10 people, including two women, were killed in the accident.
"At least five passengers were in a critical condition and were shifted to Karachi with other injured," he said.
Pakistan has one of the world's worst records for fatal traffic accidents, blamed on poor roads, badly maintained vehicles and reckless driving.