Interior Minister Aftab Sherpao said on Sunday: "Twelve people were killed and 13 injured in the explosion on the passenger bus."
The bus, which was carrying 50 passengers, was travelling through the Bolan Pass region on a regular run from the provincial capital Quetta to the eastern border city of Lahore in neighbouring Punjab province when the blast occurred.
Baluchistan police chief Chaudhry Yaqoob said "it was a timing device planted inside the bus".
Yaqoob said the injured had been taken to a hospital in Quetta.
No one has claimed responsibility for the attack.
The sparsely populated province is in the grip of a mini insurgency waged by tribesmen demanding a share of the profits from the province's immense natural resources, particularly its natural gas reserves.