The attack took place in the province's Shindand district around midnight on Tuesday, when the vehicle the victims were riding in hit the roadside bomb, said provincial government spokesman Samih Wafa.
All the victims are from the same family and were on their way from one village to another when the bomb exploded, Wafa added.
No one immediately claimed responsibility for the attack, which came hours after another roadside bombing in the province killed three policemen and wounded two others. And on Monday, another roadside bombing in Herat killed four policemen and wounded four others.
Afghan security forces are frequently targeted by insurgents and violence has intensified in the country as most international troops prepare to withdraw at the end of the year. AP