Abstract: A pressure regulating mechanism for use in a pneumatic tool includes a first relief valve and a second relief valve. The pneumatic tool includes a housing having an air intake passage, an exhaust passage and a connecting passage in air communication between the air intake passage and the exhaust passage, an air cylinder unit mounted in the housing. The air cylinder unit has an air chamber, an air passage in air communication between the air chamber and the air intake passage, and a through hole in air communication with the air passage. The first relief valve is for mounting in the connecting passage adjacent to the air cylinder unit. The second relief valve is for mounting in the through hole of the air cylinder unit. The pressure regulating mechanism maintains the air pressure inside the pneumatic tool stable.