A power output mechanism for power tools includes a main part for driving an output shaft, a switch unit, an impact unit, a pressing unit, and an operation member. The switch unit has a rotation collar for controlling positions of positioning springs. The impact unit includes an orientation wheel movably connected to the output shaft and an operation wheel secured to the output shaft. The orientation wheel includes a first toothed contact surface which is removably engaged with a second toothed contact surface of the operation wheel. A corrugated ring portion is defined on an outer periphery of the orientation wheel and limited from rotation relative to the output shaft when the positioning springs are moved to certain positions. When the positioning springs are engaged with the orientation wheel, the orientation wheel cannot freely rotate relative to the output shaft so that an impact is output.