The rate control is important to allocate bits efficiently for getting better performance, such as high quality, low fluctuation of quality and low mismatch between a target bit rate and an encoded bit rate. In this paper, we present an improved frame-layer H.264/AVC rate control scheme using enhanced complexity measure, scene-change detection, and quantization parameter adjustment for low bit rate video. Especially, our complexity measure contains not only the residual information but also the motion information. We use the motion vector difference encoded previously as motion information for low computation-load encoder. Experimental results show that Y-PSNR and bit-rate mismatch are remarkably improved and the fluctuation of Y-PSNR is similar as compared with the conventional H.264/AVC rate control scheme (JVT-G012).