Accurate servo-signal, good recording characteristics and sufficient signal-to-noise ratio are revealed to be necessary conditions for the optimum design of magneto-optical (MO) disks.' These conditions are dependent on the characteristics of the disk drive. This paper focuses on digital MO disks with a diameter of 130 mm which conform to the ISO standard. E-polarized incident light where the electric vector is parallel to the grooves is suitable for obtaining a stable servo signal and Hpolarized light where the magnetic vector is parallel to the grooves is suitable for a sufficient cross-track signal. Furthermore, a quadrilayer structure is desirable for the improvement of recording sensitivity and enhancement of the Kerr rotation.