Abstract: The visual properties of diamonds are the intuitional reflect of diamonds’ quality and value. Under different light sources, the visual properties presented by diamonds are very different. In order to comprehensively evaluate the visual properties of diamonds, the authors designed simulative jewelry store illumination light source and simulative diamond grading lamp light source. The instrument for observing diamond visual properties was used to capture diamond actual image patterns and 48 diamond samples with various cuts were analyzed. The effect of diamond cut parameters on the brilliance was studied. Results of the experiment indicated that the visual properties presented by various diamonds are quite different under the same incident angle light source even if the differences of diamond cut parameters are small. Under the different light source, the bright image pattern and brightness values of the same sample showed great differences, but the changing tendency of the brightness values along with diamond crown angle and pavilion angle were similar. Diamonds with high brightness or low brightness had similar crown angle and the pavilion angle combination.By analyzing the trend of the brilliance under the different light sources, it was indicated that the brilliance of the diamond is not positively related to the cut level of the diamond. The brilliance of the diamond with low cut level may be better than that of diamond with high cut level. That is to say, the brilliance of the diamond can not be determined based on the cut level alone, but it can be confirmed that the overall light pattern of the diamond with the higher cut level is more uniform and beautiful than that of the lower cut level. Crown angle and the pavilion angle have relevance relationship for the diamond with good brilliance. Based on the diamond crown angle and the pavilion angle combination, the authors built a linear relationship: α+ 0.167β = 44.5° (α is for pavilion angle and β is for crown angle). In conclusion, in a certain range, increasing pavilion angle and at the same time reducing the crown angle, or reducing pavilion angle and at the same time increasing the crown angle, the brilliance of the round diamond can obtain a good performance.