Abstract: The main task of the immune system is to detect and eliminate the non self materials called antigens such as virus and cancer cells which come from inside and outside of the living system The negative selection algorthm proposed which is inspired by the negative selection mechanism of the immune system is able to detect any change The algorithm is improved to make it suitable for the fault diagnosis of spacecraft Moreover,based on the improved algorithm a new method of fault diagnosis of satellite power system is proposed and its feasibility is proved by on line fault diagnosis of satellite power system successfully