A method for detecting a size of an optical disk loaded in an optical disk drive is disclosed. The optical disk drive has an optical pickup head, and the initial position of the optical pickup head is located on the outer side of a track. The steps of the method are described as follows. First, the optical disk is loaded. Then, the optical pickup head is moved from the initial position to a first position, and a laser beam is irradiated simultaneously. The laser beam reflected by the optical disk is received by the optical pickup head. According to the condition of the reflected laser beam received by the optical pickup head, the size of the disk can be determined.