This paper addressed an another scheme of distortion estimation method based on simplified inverse quantization in H.264/AVC. The distortion is calculated by the difference of coefficient between quantized transform coefficients and that of inverse. In general, these process included such transforms as discrete cosine transform(DCT), quantization, inverse quantization(IQ), and Inverse DCT(IDCT). In proposed approach, IQ as well as IDCT process are skipped because of replacing a couple of approximated formulas. Some simulation have been conducted and it showed that the PSNR was almost the same, and reduced the rate-distortion optimization(RDO) mode decision time of 8~15% in comparison with conventional method.