A ratchet wrench includes a handle, a driving head, an engaging member, a spring and a controlling knob. The handle has a receiving groove, a transverse hole in one lateral side of the handle, and a longitudinal hole in a bottom of the receiving groove. The driving head has a connecting portion pivotally connected to the handle. The engaging member is movably received in the longitudinal hole, and has a guiding cutout in a lateral side thereof. The spring is interposed between a bottom of the longitudinal hole and the engaging member. The controlling knob has an abutting block received in the guiding cutout of the engaging member. The abutting block is substantially oblong in shape such that rotational movement of the controlling knob drives the engaging member to be disengaged from the driving head.