Abstract: In order to measure inhomogeneity of optic material accurately,an optical wedge was prepared at a small angle less than 0.117°.Then its inhomogeneity was measured with a Fizeu interferometer.This method was proved to be correct by simulation and experiments in which the inhomogeneity of fused silica was measured,and the peak to valley value of fused silica was 4.33×10-6,and the root mean square value was 0.862×10-6,and the accuracy of this method was less than 4.8×10-7. The results show that the light reflected either from the front surface or from the near surface can be distinguished with this method.