The characteristics of amplitude modulation of a reflective liquid crystal on silicon (RLCOS) are discussed in this paper. The 2×2 Jones matrix methods is used to analyze amplitude modulation characteristics. The system is constructed for testing of amplitude and phase, in which He-Ne laser is used as the light source, the wave front amplitude is measured by the interferometer. The RLCOS display device as a mirror is used as a spatial light modulator and its driving circuit is controlled by a computer, digital data are sampled by CCD. The amplitude modulation characteristics of 1024×768 RLCOS display device are gotten. Comparing with the without sharped laser beam profile, the results show that under selected polarization states of input and output, RLCOS displays can be used as amplitude modulator.