This paper points out some defects in the techniques used in H.264 rate control and presents two new techniques to improve them. The improved scheme has the following main features: 1) the bits allocated to each P-frame is proportional to the local motion in it, i.e, more bits are allocated to a frame if the local motion in it is stronger; 2) the quantization paramter (QP) calculation is based on a simple encoding complexity prediction scheme, which is more robust and simple than the quadratic model used by H.264 in low bit rate video coding. Simulation results show that compared to rate control scheme in H.264, the improved scheme has significantly improved R-D performance (up to 1.29dB).