In this paper, we propose a macro-block (MB) level rate control algorithm for low delay H.264/AVC video communication based on the ρ domain rate model. In the proposed algorithm, an exponential model is used to characterize the relation between ρ and the quantization step (Qstep) at the MB level, with which the quantization parameter (QP) for a MB can be obtained. Furthermore, a switched QP calculation scheme is introduced to obtain the QP for each MB to avoid large deviation of the actual frame size from the target bit budget. Compared with the original ρ domain rate control, the proposed method can achieve better video quality and improved bit-rate accuracy. Meanwhile, the computational complexity is also significantly reduced.