The incident took place in Mastung district when the buses were taking pilgrims from the province's Taftan town to Quetta city.
One of the buses, carrying 43 passengers, was completely gutted, while another was partially damaged in the targeted attack, when the attacker rammed his explosives-laden vehicle into the bus, killing 19 pilgrims and injuring 25 others, said Mastung Deputy Commissioner Tufail Baloch.
None of the terrorist organizations claimed the responsibility for the explosion.