A ratchet wrench includes a head with a ratchet wheel rotatably received therein. A positioning unit is located in the head and is cooperated with a pawl which is engaged with the ratchet wheel. A collar is located in the head and has two notches defined in a peripheral wall of the collar. A curved groove is defined through the collar. Two toothed racks are formed along the groove. Two clamp members are located in the ratchet wheel which has two guide slots. Each clamp member has two guide rods which are located in the guide slot corresponding thereto. An engaging unit extends through the groove and is pivotably connected to a lever to securely position the clamp members when clamping an object. The two clamp members are adjusted to securely clamp the object by rotating the collar, and the object can be rotated by the ratchet wrench.