A ratchet screw driver includes a holder, a toothed wheel, two pawls, a direction switch and a ring. The toothed wheel, the pawls and the ring are inserted in the holder. The pawls are biased from each other. Each of the pawls includes a boss and a toothed face. The direction switch is connected to the holder, movable between two positions relative to the holder, and formed with two bosses. The ring includes two cutouts and two hooks. The cutouts receive the bosses so that the ring is rotatable with the direction switch. One of the hooks is engaged with the boss of one of the pawls to firmly engage the toothed face of the remaining one of the pawls with the toothed wheel when the ring is moved to one of the positions by the direction switch.