For Bose-Einstein condensates in one dimensional optical lattice, the phase fluctuation is a significant quantity in the investigation of quantum phase transition. A method to measure the phase fluctuation was investigated by comparing the contrast of interference peaks obtained from experimental absorption images and simulated data sets. Phase variances of condensates in optical lattices with different depth were measured by this method. The phase fluctuations increase from 0.29π to 0.69π when the depth of optical lattice varies from 5.6ER to 30.7ER.