Six people died on the spot while three others died during treatment in a local hospital, according to Nepal Police.
The accident happened at Daunne area of Nawalparasi along the East-West Highway.
The fully load truck with Indian registration number collided with the bus heading towards Dang from Kathmandu, police said.
Seven people traveling on the bus sustained injuries in the incident.
Among the injured was truck driver Byas Sharma, 51, of Devariya area of Kanchanpur district, Uttar Pradesh in India, police said.
According to the preliminary investigation, the accident took place due to brake failure of the truck.