Disclosed is a hydraulic stopper of a shock absorber. The shock absorber includes a cylinder and a piston rod having a piston valve. The stopper includes a closing member at one side of the piston rod to protect the piston valve, a free piston movable between the closing member and the rod guide to define a fluid passage with the piston rod, a sealing member between the free piston and the cylinder, and a rebound spring between the free piston and the rod guide to elastically support the free piston such that the free piston comes into close contact with the closing member to block the fluid passage when the piston rod moves excessively upward. The stopper may generate buffering effect using oil pressure without being restricted by the length of a rod guide, applied to a general shock absorber, and permit components thereof to be shared with the general shock absorber.