Based on vertical Fizeau interferometer for radius of curvature (ROC) measurement system, a simple null position power compensation algorithm for defocus at cat′s eye position and confocal position is proposed to improve the measurement accuracy of ROC. The linear relationship between defocus of the wave- front aberration and the offset of tested surface is theoretically analyzed. And the theory line slope between the two is got. The results show that between power value at cat′ s eye and confocal null position and tested surface position offset is the linear relation. And the fitting line slope of experiment is consistent with the theoretical value. Furthermore, radius of curvature measurement precision is improved by an order of magnitude with using the defocus compensation algorithm when the tested surface with some defocus at confocal position.