Region of interest (ROI) encoding is an important feature of SVAC. The efficiency of image processing can be increased and the bit rate reduces greatly because of ROI, but the bit rate will fluctuate along with complexity of encoding frames. Therefore, availability of communication channel is inefficient. There has been no rate control algorithm recommended by SVAC so far. A rate control algorithm is thus provided for ROI based on SVAC. The bit rate of ROI and back region are allocated by calculating the complexity of image, and the bit rate of ROI will be allocated preferentially. Meanwhile, a virtual buffer is designed. The occupancy of virtual buffer is calculated to adjust the quantization parameter (QP) of ROI and back region of every encoding frame in time. Experiment results show that bit rate is stable in communication channel whose availability is improved. Besides the image quality of ROI is smooth and excellent.