Abstract: The image sensor in the optical communication for satellite may produce noise due to the space irradiation damage, which leads to the deviation in determining the light spot centroid. Based on the irradiation test data for typical CMOS devices, different sizes of simulated defect spots are used for calculating the centroid deviation value by a grey-level centroid algorithm, and the impact on the tracking & pointing accuracy of the system is analyzed. It is shown that both the amount and the position of defect pixels caused by the irradiation contribute to the spot centroid deviation; and the larger the spots, the less the centroid deviation will be. Some suggestions are made with regard to the constraints for the spot size selection with consideration of the space radiation damage.