The graininess of the color film is one of its important quality factors,and it is usually indicated by the RMS granularity. Actually,the RMS granularity does not always represent the graininess feeling of human on the color print,for only one gray channel (white light) densities are sampled in it. In this work,red,green and blue channel densities are all counted,resulting in three granularities. Then they are transformed into three printing granularities involving properties of the printing machine and the color paper. Finally they are resembled in a print grain index involving characteristics of human eye as well as psychology. The print grain index represents the graininess on the color paper that a color film is printed. Some Kodak films assample films have been measured and observed. The results give the measuring accuracy and coincidence between the print grain index values and the perceptible graininess on the prints, proving the efficiency of the method.