A driving head of a wrench includes three engaging surfaces and three reception concavities alternatively defined in an inner periphery thereof. A recess is defined in a center of each of engaging surfaces and located between first and second engaging portions. The last reception concavity has one of two insides connected to the second engaging portions next to it, the other insides of the last reception concavity is connected with a third engaging portion. The recesses and the reception concavities are sized to accommodate protrusions of a star-shaped nut. The reception concavity is sized to accommodate one of protrusions of a star-shaped nut. The adjacent first and second engaging portions have a common tangent line and three respective common tangent lines are located on three sides of a hexagonal object. The wrench is able to drive the object in two opposite directions without removing the wrench from the object.