Cooling system is a subsystem of a power plan that has an indispensable effect on efficiency of power plant. Air Cooled Condenser (ACC) system as a cooling system, is usually used in many power plants. Steam duct as a part of ACC conveys the steam flow with high velocity form steam turbine exhaust to head of tube bundle which is located at height of 25 to 30 meters. In this paper, pressure drop in steam duct is calculated using computational fluid dynamic method. Then, in order decrease pressure drop and consequently increase ACC efficiency, steam duct configuration is modified. The steam flow is modeled as an incompressible, turbulent and single phase flow. The solution strategy is based on SIMPLE and Segregated Implicit algorithm and the k-ε RNG model for turbulence are employed. The second order upwind differencing scheme are applied to discrete governing equations. Numerical results confirm reducing of total pressure drop in modified diffuser comparing with primary geometry.