Diamond color grading has been firstly designed to form a set of mature grading system by Gemological Institute of America. Traditionally, diamond color grading method is to observe the sample visually and compare it with a set of standard colour master stones. The classification results are mainly affected by the testing environment, size of diamond, inspectors and so on. Then, grading deviations will always exist for the same sample, even if it is tested by the same person in the same environment.To avoid the above problems, it has been a hot research topic in diamond field to use an excellent instrument to evaluate the color of diamond. However by now, there have not yet been any instruments which can completely meet the requirement for the grading of diamond. This paper gave an interesting method for quantitative evaluation of diamond color, and the micro zone measurement was applied and proposed to avoid the effect of size variation results in color grading of the diamonds. The principle of spectrophotometer was used in the method, which consisted of a sample system, a micro positioning display system, an automatic acquisition and calculation system. D65 light source was employed and its color temperature was (6 500±200) K. The color signal acquisition was realized by CCD optical fiber spectrometer with high speed, high resolution and large pixels. Several sets of master diamonds with the national(China) standard certificate and with different sizes and different colors were employed to test. The results showed that the method could obtain the index of the color of the diamond accurately and could transform to the degree of diamond grading color system. By using the micro area accurate positioning method, selected the common features of the diamond pavilion measurement area, which could eliminate the errors of the same color grade diamond due to the size. The method is a potentially useful technology for diamond color grading.