Masked robbers threatened a petrol station cashier with a gun before forcing him to hand over the takings.
The raiders targeted the BP petrol station in Cherwell Drive, Marston, Oxford, at about 0610 GMT.
Both robbers were white, were wearing black jackets and had scarves over their faces. The gunman was about 5ft 8in and his accomplice about 5ft 6in.
Officers carried out door to door inquiries and patrolled the area to reassure residents.
Det Insp Steve Raffield of Thames Valley Police urged anyone with information to contact police.