Color rendering feature of certain device or process can be described more specifically if the color gamut can be described quantitively which is difficult to achieve due to the color samples dispersed unorderly in color space. In this paper, a method of comparising color gamut quantitively is brought forward based on spacial dissection theory. A cetain color gamut can be represented by a certain and unregular three dimentional cubic in the Lab color space. Marking out the coordinates’ data on the surface of this cubic, means the outline of the color cubic is obtained. These special coordinates’ data are those color spots which possess strong saturation, and this feature facilitates the process of outlining the color cubic. Based on this feature, according to the Delaunay theory applied in three dimension, the dispersed data in Lab color space produces tetrahedral network, which means the irregular color cubic can be divided into tetrahedrons, whose volume can be calculated via its coordinates’ data（the value of Lab）, and consequently, the whole volume of color gamut can be be finaly obtained by these divided tetrahedrons. Experimental results show that, this method can be applied effectively for gamuts comparison quantitively, further more,by choosing certain color spots in certain area of the color space, more accurate calculation and comparison can be obtained.