The bomb, hidden in a motorcycle, was detonated by remote control as the bus was passing through Mastung district, 50 km south of the provincial capital of Quetta.
Officials confirmed the death of six persons. Over two dozen people were injured.
Nearly 50 people, including women and children, were in the bus.
The condition of three of the wounded was critical, officials said.
The injured were taken to a nearby hospital and those in a critical condition were later shifted to Quetta.
Footage on television showed that the bus was extensively damaged by the blast.
Police officials said the intended target of the bombing might have been a bus with Shia pilgrims that passed the area minutes before the explosion.
No group claimed responsibility for the attack.
Balochistan has witnessed a string of sectarian attacks that targeted the minority Shia community.
Scores have died in these attacks.
Islamabad, Jun 11 (PTI) Six persons were killed and dozens injured when a bomb went off near a bus in the restive Balochistan province of southwest Pakistan on Monday, officials said.
First Published: Monday, June 11, 2012, 10:35