The properties of the color film and the affecting factors when measuring the print grain index value have been analyzed. The film measuring point of print grain index should be made according to its light-sensitive characteristics, being the density corresponding to the 0.74 gray density of color paper when printing; the two main factors affecting print grain index accuracy are contrast amendment factor a of the color paper and cover power efficiency ECP of the color film. The methods of obtaining a and ECP are found, and these two efficiencies of several films have been measured. Finally, the print grain index amendment arousing from the variation of a(ECP) is calculated.