The explosion occurred near Kolachi in Dera Ismail Khan district, which borders the South Waziristan tribal region.
The police van was on a routine patrol at the time of the attack.
Officials said the chief of Kulachi police station was among the injured.
The attack came as Pakistan celebrated Eid-e-Milad-un-Nabi, the birth of Prophet Mohammed amid strict security measures.
No group claimed responsibility for the attack though such incidents are usually blamed on the banned Pakistani Taliban.