A cool air supply duct of a refrigerator includes a vertical duct unit installed in a freezing chamber or a refrigerating chamber of the refrigerator so that the length of the vertical duct unit can be adjusted in the vertical direction; and horizontal duct units installed on the vertical duct unit so that the length of the horizontal duct unit can be adjusted in the horizontal direction and provided with cool air discharge holes formed therethrough for discharging cool air. Since the cool air discharge holes are changed to positions desired by a user, the cool air supply duct cools a designated space of the freezing chamber or the refrigerating chamber of the refrigerator to a temperature lower or higher than those of other spaces, thus increasing convenience in using the cool air supply duct and improving the cooling efficiency of the cool air supply duct.