In this paper, we discuss the registration of two interfering SAR images based on the least square theory. A new measure is proposed to compute sum of phase difference of two SAR images, and is minimized to find the optimal registration parameters. In one of the previous studies, images from two satellite passes was registered by computing statistical correlation function between the two images over discrete pixel offset, then interpolating the correlation function to find its minimum position. Another approach was to interpolate the second image on a subpixel basis, evaluate the average fluctuation function of the phase difference image, then adjust the registration parameters according to the change in the average fluctuation function, and interpolate the second image again. The process is repeated until the average fluctuation function reaches its minimum. In our approach, registration is done according to the least square theory, computing sum of squares of phase difference of two SAR images. We can find optimal registration parameters when the sum is minimized. The least square registration is a method of subpixel accuracy and can get registration's specific accuracy. The registration's accuracy is 0.05 pixel to 0.07 pixel in our test.