Abstract: An impact screw driver includes a tubular handle, an impact rod slidable axially within the handle, and a handle cap mounted detachably on the top end of the handle. A driving rod rests on the top end of the impact rod and has an open top end and a lower end spring seat, on which a torsion spring is provided to bias a driving head to move upward. The impact rod and the driving rod are both guided to move axially in the handle. A sleeve head connects a tool bit to the driving head. A compression spring is sleeved on the driving head and placed over the driving rod so as to bias the driving rod and the impact rod to move downward. A ball is retained on the wall of the driving rod. A cam slot is formed in the driving head so as to receive the ball slidably in the cam slot. The cam slot is shaped so as to convert axial movement of the driving rod into rotational movement of the driving head.