It is always attempted to reduce the costs of comminution in mineral processing plants. One of the difficulties in size reduction section is not to be designed properly. The key factor to design size reduction units such as crushers and grinding mills, is ore’s work index. The work index, wi, presents the ore grindability, and is used in Bond formula to calculate the required energy. Bond has defined a specific relationship between some parameters which is applied to calculate wi, which are control screen, fine particles produced, feed and product d80.In this research work, a high grade copper sample from Miduk copper concentrator was prepared, and its work index values were experimentally estimated, using different control screens, 600, 425, 212, 150, 106 and 75 microns. The obtained results from the tests showed two different behaviors in fine production. According to these two trends the required models were then defined to present the fine mass calculation using control screen. In next step, an equation was presented in order to calculate Miduk copper ore work index for any size. In addition to verify the model creditability, a test using 300 microns control screen was performed and its result was compared with calculated ones using defined model, which showed a good fit. Finally the experimental and calculated values were compared and their relative error was equal to 4.11% which is an indication of good fit for the results.