In order to enhance the angle measurement accuracy in passive radar seeker (PRS) restrained by weight volume, a novel Extended Kalman Filter (EKF) using missile-to-target relative state is proposed in this paper. Missile-to-target relative position was described by several relative state variables, including range, line-of-sight azimuth and elevation angle. Missile-to-target relative motion model was then established by using velocity measurement of Inertial Navigation system (INS). This approach can fully take advantage of onboard INS information, thus offering favorable PRS measuring error suppressing. It has practical application in enhancing missile strike precision, attacking performance as well as the ability of anti-target losing. The approach proposed is validated by the simulation in a representative scenario.