Computer Generated (CG) images on a CRT monitor cannot be accurately reproduced on a hardcopy output when the gamut of the CRT monitor exceeds that of the printer. Such cases require that the out-of-gamut colors be mapped inside the output device's gamut while preserving the appearance of original image as much as possible. This technique is called gamut mapping. While several gamut mapping techniques have been suggested for the natural images, techniques for CG images have rarely been discussed.In this paper, a three-dimensional gamut mapping technique for CG images is considered. A weighted color difference equation for lightness, chroma, and hue, along with normal CIE 1976 color difference equation were used in the gamut mapping. Visual experiments were performed to evaluate the importance of these attributes in image appearance. Results showed that human visual system is most sensitive to lightness change, and least sensitive for chroma change. Surprisingly, for CG images, hue change was less noticeable than the lightness change.