With obvious advantages comparing to gimbaled seekers, strap-down seekers attract more and more attention from military of different countries. Since strap-down seeker can not measure the line-of-sight (LOS) angular rate directly, the problem of LOS angular rate extraction is solved firstly, which generally has two methods. One method is to get the LOS angular rate by solving the differential of LOS angle versus time, the other is to estimate the LOS angular rate by Kalman filter. The models of these two methods was built and simulations of them were made respectively. The simulation result shows that, the second method is better than the first one. The time delay of LOS angular rate estimated by Kalman filter is relatively shorter than that obtained through differential network. With the measurement noises at the same level, the output LOS angular rate noise of the second method is smaller than the first one.