A theoretical analysis was conducted to explore the flow structure adjacent to rough walls and its influence on flow friction.The flow friction resistance coefficient was found not only dependent on the relative roughness，but also the geometrical and hydrodynamical features，such as roughness elements density and flow reattachment length.It was irrational that the effects of roughness on the coefficient could be neglected with the criterion of the relative roughness being below 5%.The equivalent treatment of flow area constriction was made，and modified formula and resistant deviation coefficient η were introduced to correct the reduction value of the duct diameter as the function of relative roughness ε/d，roughness element density parameter A and Reynolds number Re.The resistance deviation coefficient increased with increasing A and Re，and the effects of roughness could be ignored at very sparse roughness element distribution or at low Re.