Evaluation and improvement of sweep efficiency are important issues of miscible displacement process. Miscibility is controlled by physical dispersion during miscible processes. It comprises of two parameters: diffusion and mechanical dispersion. Unfortunately this parameter is usually ignored during simulating the miscible processes which would be considered as an enormous source of error. Results show the existence of three regions at which each component of physical dispersion controls the miscible CO2 injection. For dispersivities between 0.123m to 0.175m the miscible process is controlled by physical dispersion mechanism while dispersivities lower than 0.123m specifies a region controlled by diffusion in the field case. The third region is the region for dispersivities greater than 0.175m and the main parameter affecting the miscible process in this region is mechanical dispersion. This study discusses the the effect of this parameter on recovery, mixing zone length and breakthrough time of injecting CO2 in a natural fracture reservoir in Iran.