The bus was carrying 38 passengers. Eleven passengers who were seriously injured, were admitted to hospitals at Murbad, Otur and Ale-Phata, police said.
The accident site is around 154 km from here. The bus was on its way from Thane to Ahmednagar.
"The truck, carrying wooden planks, collided with the bus, knocking it down the valley," MSRTC General Manager (Traffic) Suryakant Ambawdekar said.
Authorities sought the help of trekkers for retrieving the bodies from the bus which was mangled badly.
Maharashtra government announced compensation of Rs two lakh each for the kin of the deceased. Chief Minister Prithviraj Chavan said the compensation would be given from the Chief Minister's relief fund. The injured will be given Rs 50,000 each.
MSRTC chairman Jeevan Gore said the corporation would give Rs 3 lakh each to the families of the deceased.