A portable power working machine includes a drive unit disposed in a motor housing connected to a proximal end of an operating arm. The drive unit rotates a cutting blade at a distal end of the operating arm. The motor housing has an inlet and an outlet for cooling air in front and rear sections, respectively. The drive unit includes an electric motor disposed near the front end of the motor housing, a cooling fan disposed near the rear end of the motor housing and connected to an output shaft of the electric motor, and a heat sink disposed between the cooling fan and the electric motor so as to extend from the electric motor. The cooling air drawn in through the inlet by the cooling fan is guided through a gap between the drive unit and an inner surface of the motor housing and is discharged through the outlet.