This paper discusses the characteristics of phase-only modulation using a reflec tive liquid crystal on silicon(LCOS). The 2×2 Jones matrix method is used as a computational tool to theoretically analyze phase modulation characteristics. Th e system has been constructed for testing of amplitude and phase, in which a He- Ne laser as the light source, testing the wave front phase using an interferomet er. The LCOS displays as a liquid crystal spatial light modulator and its drivin g circuit is controlled by a computer, digital data are sampled by CCD. We have tested the phase modulation of 1024×768 LCOS displays. Given the state of input and output polarizations ,the result of testing our theories by experiment sho w consistency ,so LCOS displays can be used as phase-only modulator.