In this research, we studied the surface roughness affecting on the pressure drop in a flat plate and a pipe. Based on CFD(Computational Fluid Dynamics) technique, using commercial codes such as ANSYS-FLUENT, the Darcy frictional coefficient is shown precisely to be changed with a range of various surface roughness ratios as well as Reynolds numbers. Surface roughness is a key parameter to determine the friction at the wall of pipe for a given Reynolds number in turbulent flow. In the present investigation, SST(Shear Stress Transport) turbulence model is used for the highly turbulent flow. From the preliminary examples, experimental and theoretical correlations like flat plate solution and Muddy chart formula are compared with the result of computation, and the good agreement is shown for the wall friction data. Through this research, feasibility and accuracy of the present CFD technology for the prediction of the pressure drop are checked for the primary problem of the U-tube used for the thermal-hydraulic system of PWR(pressurized water reactor) in the application of nuclear engineering.