Abstract: In spacecraft formation flying application,relative attitude of the deputy spacecraft with respect to the chief spacecraft needs to be determined.Relative attitude determination based on extended Kalman filtering(EKF) is presented.The modified Rodrigues parameters(MRPs) are here adopted as the attitude variables,which has no redundancy and has the largest unsingular range.Attitude sensors include star tracker,laser rendezvous radar and gyro,and measurement equations of the sensors are given.The state equation of the EKF is obtained from the relative attitude dynamic equation.Simulation results based on the given EKF model showed that the state variables of the EKF converge within at most 300 s,and that,during the steady state,the estimate error of MRPs is within 10-5 showing the validity of the proposed method.