The family was travelling in the Shindand district of Herat province late yesterday when explosives ripped through their vehicle, Abdul Hameed Noori, the district chief, told AFP.
"Nine members of the family including two women, five children and two men were killed on the spot," he said.
Two people were wounded and taken to a nearby hospital.
Sameh Wafa, a provincial spokesman, confirmed the incident to AFP.
Roadside bombs are commonly used by Taliban insurgents to target Afghan and foreign forces but often cause injuries to civilians.
Also Read
Afghanistan is in the middle of presidential elections, with former foreign minister Abdullah Abdullah and ex-World Bank economist Ashraf Ghani due to compete in a head-to-head run-off vote next month.
Both candidates have promised to bring peace after decades of conflict.
But they will have to tackle security without US-led NATO combat troops, all 51,000 of whom will pull out by the end of this year.