The truck hit the landmine in Chattar area of Dera Murad district killing five people on board and critically injuring another, officials said.
"The blast was intense and four people died on the spot while the fifth person expired later in hospital," he said.
"The injured person is also in critical condition right now," he said.
District police officer Mahmood Jamali said the landmine was apparently planted by separatists who are active in the area.
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Although no group has claimed responsibility for the attack, but a police official said insurgents carry out such attacks against security personnel and installations.
"I don't think the pick-up truck was the intended target as it was carrying normal passengers," he said.
On Saturday, a government official said that insurgents had blown up two power transmission lines near Sibi in the province causing a major blackout in the cities of Karachi and Quetta and other parts of the country.