It happened at approximately 8 p.m. Sunday at 7-Eleven on North Green Bay Road.
The store's manager was trying to assess the financial damage Monday.
"I'm always extra cautious. We all are. According to what he was saying...we don't know how much. When I do the report tonight, we'll find out how much they took," said Moe Moosa, the store manager.
Moosa also says there have been a number of other armed robberies in the area recently,-including one at a nearby Taco Bell restaurant.
There was no word from police on whether the two incidents were connected.