A surgical stapler which includes a tool assembly having a pair of opposing tissue engaging surfaces for deforming a plurality of surgical fasteners through and fastening tissue is disclosed. The surgical stapler includes a housing having a fixed handle and a movable handle mounted to the housing and selectively movable relative to the fixed handle from a first position to a second position to actuate the clamping of tissue. The stapler further includes a drive assembly including a motor, a power source and a coupling member. The motor engages the coupling member, wherein movement of the movable handle to the second position activates the motor, which advances a firing shaft and forces a firing piston into the tool assembly to deform the surgical fasteners through and fasten tissue.