The bomb was triggered by remote control as the van was going from Nowshera to Mardan.
Four prisoners were being taken in the van to Mardan for an appearance in court.
Two policemen, who were seriously injured, were taken to a hospital in Mardan city. One policeman and four prisoners sustained minor injuries, officials said.
The prisoners were later taken to a jail in Risalpur city and locked up, the officials said.
No group claimed responsibility for the incident though such attacks are usually blamed on the Taliban.