A constant-quality rate control system and algorithm for multiple encoders with a single video source are disclosed. The system comprises a base encoder, a region of interest (ROI) detector, and at least one ROI encoder. The base encoder encodes a base video sequence which is a down-sampled version of a video sequence and sends the base video sequence to a multiplexer. The ROI detector extracts an ROI video sequence from the original video sequence. The ROI encoder encodes the ROI video sequence and sends it to the multiplexer. The ROI encoder uses information obtained during encoding of the base video sequence to improve the encoding of the ROI video sequence. The information of the base video sequence are used to better control the bit-rate and the quality of the ROI video sequence.