In order to evaluate the performance of a reflective 256 pixel×256 pixel phase-only liquid crystal spatial light modulator (LC SLM) from American BNS Company, interference measurements are proposed on the characteristics of phase modulation using a Twyman-Green interferometer. Experimental results indicate that the relation between phase and gray level is nonlinear. A linear look up table is established by inverse interpolation to correct the nonlinearity for this device. The nonlinearity decreases to 2.85% from 17.23%. The phase difference between the centre and the edge is 0.22π on the equal gray level and the root-mean-square error of phase modulation for the centre is 4 times of that for the edge, which shows the phase modulation for this device is nonuniform and the phase error for the centre should be corrected in wave-front control application.