The bus was traveling from Wadh to Dadu when it crashed on the M8 in Khuzdar district.
Fifteen people were killed on the spot and three others died shortly after their departure, rescuers said.
Rescuers rushed to the scene and rushed the injured and dead to Teaching Khuzdar Hospital.
The Deputy Commissioner of Khuzdar, Bashir Ahmed, confirmed the dawn and the first death of 18 people.
The condition of some of the injured is said to be critical.
The bus picked up the returning pilgrims after the annual Sufi festival Abdul Qadir Naqshbandi in Wadh.
Officials said the coach lost his strength and rolled over because he was too fast.
Road accidents are common in Pakistan and are mostly caused by reckless driving, illegal traffic, and bad roads.
On May 31, at least six people were killed and 30 others injured when a bus they were traveling in crashed into a bridge in Khanewal in Punjab province.
Similarly on Friday, last year in March a passenger bus overturned near the crossing of Shah Noorani in Lasbela, Balochistan, killing at least five family members and injuring 24 others.