Police are investigating an alleged armed hold-up at the 7Eleven petrol station on Pinjarra Road in Mandurah on Thursday afternoon.
Police allege a man entered the store about 12.20pm armed with a weapon and robbed the store, with staff handing over money.
They say he fled on foot after taking some cash.
The petrol station is currently blocked off at both entrances by unmarked police cars, with patrols being conducted by officers.
Regional Investigation Unit officers are investigating with assistance from Mandurah Detectives.
Mandurah Detective Senior Sergeant Trevor Troy said the search for the accused was ongoing.
"We will assist the unit into the night to find the offender, if needs be," he said.
"We are currently using CCTV. From that we will get a good idea of him."
Senior Sergeant Troy described the accused as a man in his early 20s, with a slim-to-medium build and olive complexion.
He said the accused was wearing a white top, red shorts and dark shoes. The alleged armed robber covered his face with the hood of a windcheater.
Anyone with information should contact Crime Stoppers on 1800 333 000 or file a report at crimestopperswa.com.au.
