Abstract: A set of experimental system of magneto-optic Kerr effect was set up mainly by using photo-elastic modulator and lock-in amplifier. The system was used to measure the magnetic properties of magnetic materials, especially for magnetic thin films. The longitudinal magneto-optical Kerr effect was used to investigate the magnetic properties and magnetic reversal. Transverse and longitudinal magnetic fields were applied in the plane of the samples in order to measure two perpendicular magnetization components in-plane. Co/Cu/Co sandwiches deposited on different substrates were prepared by high vacuum magnetron sputtering. From Kerr loops, the different of structure between buffer and Cu layer induced the difference of structure between Co down-layer and Co up-layer, which made the coercive force difference and magnetization reversal.