The bomb was triggered by remote control while the van was passing through *Badabher, a suburb of the capital of Khyber-Pakhtunkhwa province, this morning.
A sub-inspector was among the dead, police officials said.
The blast destroyed the police van, the officials said. Two kilograms of explosives were used in the attack.
Security forces cordoned off the area and launched a search operation.
No group claimed responsibility for the incident though such attacks are usually blamed on the banned Tehrik-e-Taliban Pakistan.