As the image quality of color and monochrome electrophotographic printers has improved, they are beginning to be used for graphic users. Printer output images that contain gradations have been increasing, and smooth reproduction of gradation is one of the important subjects of image quality. This paper describes an evaluation method of gradation by using decomposition into Lightness and Chroma based on micro colorimetry with a flatbed scanner. In order to evaluate the reproducibility of gradation, difference value from a desirable gradation is computed, and a perceptual threshold value is determined that can be used as a basis for judgment of detecting difference value. The desirable gradation is derived from compensating for measured gradation by the transfer function of human visual perception. A high correlation between the total of difference value that is greater than a perceptual threshold value and psychological score of smoothness of gradation, is achieved.