When a color video system with highly accurate color reproduction is designed, a good measure for evaluating the image quality including the color reproduction is required. Especially, in codec of motion pictures a good measure is strongly desired to determine a proper compression rate (bit rate). In this paper, we focus on the image quality of motion pictures compressed by H.264/AVC codec which is receiving increased attention. In this study, we investigated the usefulness of S-CIELAB. At first, S-CIELAB color difference calculation was applied to the frames of motion pictures after H.264/AVC codec. The performance was better than CIELAB color difference but not satisfactory. We then limited the region of calculation of CIELAB color difference to the smooth regions where compression error tends to attract attention of observers. Experimental results showing that the modification is promising are obtained.