A method is disclosed for adjusting the rotational speed of a disk drive. According to the method, the proportion of time which the disc drive is reading data from the disc during a unit time T is calculated. Then, the increase of the rotational speed of the motor is avoided if the error rate Ek that a disc drive reads data during each of r consecutive units of time periods T is not less than a criterion Q. Otherwise, the rotational speed of the motor of the disc drive is increased if the proportion of time Pk that the disc drive spends reading data from the disc is greater than a threshold H during each of m consecutive units of time period T, and the rotational speed of the motor of the disc drive is decreased if the Pk is less than a threshold L during each of n consecutive units of time periods T.