We propose a transcoding method of H.264 coded bitsteam to control the picture quality dependently on the interest region. In the proposed method, first we find the model of quantization step-size and bitrate. Then a classification method according to the subjectively interest region within a video sequence is suggested. Also we propose a method that assigns a specific quantization step-size differentially according to the interest region within a video. In general, the subjective picture quality can be increased by applying the quantization step-size as a small value relatively for the interest region compared with the other regions. From the simulation, we show that the proposed method can make better subjective picture quality relatively in parts of interest region.