Abstract: Cut ratio plays an important role in evaluation of diamonds. People usually use diamond lamp to observe and choose diamonds which are better cut. However, in different lighting angles, we will observe diamonds with different brightness. In order to find out the most suitable angle to sort diamonds of better cut grade, the authors simulated the brightness of round brilliant diamond(crown angles from 27° to 38° and pavilion angles from 39° to 43°) under illumination source such as diamond lamp in different lighting angles. When the incident angle of diamond lamp is between 20°-25°, the patterns of better cut areas are uniform and the values of brightness are higher than bad cut areas and the upper half is brighter than the bottom half. The preferable position to observe diamonds is making diamonds placed in the slots with their table facets up, with the sorting board flat on the table. Rest one's forehead against the lampshade with line of sight vertical to the table facets of the diamonds. Use this method to sort diamonds, and the diamonds with higher brightness, uniform brightness distribution mainfest themselves as the upper halves brighter than the bottom halves.