Abstract: Quantization interval determining method and circuit are provided. The quantization interval determining circuit has a quantizer matrix calculator, a macroblock classifier, a reference quantization interval storage, a block classifier, a bit table storage, a histogram calculator, a frame bit generation predictor, a reference quantization interval adjuster, a target bit allocator, and a bit rate controller. In the quantization interval determining method and circuit, a video signal is classified based on human visual sense characteristics, a reference quantization interval suited for each class is determined, a target bit is adaptively allocated, considering distribution of the video signal, to determine a quantization interval compatible with a given transmission speed, simultaneously with the reference quantization interval determination, and then a final quantization interval is determined.