ROI coding can effectively improve the subjective quality of image. There are some known disadvantages in existing ROI coding methods. This paper presents a new ROI coding method based code block selection mechanism. The method is to separate ROI and background data, and to place them to individual code blocks, ROI code block and background code block, which are put to entropy encoder independently. In the bit-stream allocation phase, a weight is assigned to the distort-length slope of ROI code block, which is intended to change the order by which the respective code blocks are assembled to stream. To transfer the information of the separated ROI and background, each original code block is duplicated logically in packet header. By assigning the same value as original code block to the number of bit planes of logical code block which has no data actually, the length of tag tree code is reduced. Because the separated code block can be assembled flexibly, the method enables the dynamic stream allocation according to the content of image.