Abstract: Phase Adjustment by Contrast Enhancement (PACE) algorithm is a new nonparametric autofocus algorithm, which can extract effectively high frequency phase errors from Synthetic Aperture Radar (SAR) images. PACE algorithm searching the best parameters by optimization method needs much heavy computation load because it takes directly the phase correction vector as variables to be estimated. To reduce the computation load, an improved PACE algorithm is proposed in this paper, which is called Interpolated PACE (IPACE) algorithm. The IPACE takes the contrast function as object function and some variable of the phase correction vector as parameters to be estimated The IPACE algorithm needs much less computation load than the PACE, and the image quality achieved by IPACE is almost the same good as the quality through PACE. The results of focusing the real SAR data show the method is a good robust autofocus algorithm.