In many applications, video sequences must be transmitted constant bit rate channels (CBR). Therefore, rate control has to be used to regulate the variable bit rate of coded stream. In this paper, we propose an effective bit-rate control algorithm in H.264/AVC for CBR transmission. Our algorithm jointly decides mode and quantization using rate distortion optimization (RDO) technique. And to reduce computational complexity, the proposed algorithm uses modified version of rate and distortion model calculated from the histogram of DCT coefficients. Since the proposed algorithm induces the rate and distortion model from the current macroblock instead of the previous coded frame or macroblock, it can effectively prevents the fluctuation of the output bits especially when scene change occurs, while keeping the quality in same level. And it can not only achieve the improvement of the rate control but also meet the target bit rate more accurately.