A power stapler includes a barrel with a cylinder received therein and a piston is movably received in the cylinder. An end cap is connected to the rear end of the barrel and a frame is movably engaged with the inside of the end cap. A seal part is engaged with an annular space in the outside of the frame and contacts the cylinder. A plurality of passages are defined through the frame and communicate with the annular space so that the seal part is securely positioned because the pressure in the passages is lower than the outside of the seal part. A fixed member is connected to the end cap and a pad is engaged with the fixed member. The pad has notches in the distal end thereof which contacts the piston so that air trapped between the pad and piston is released via the notches when the piston moves back to the original position after a shot.