Rectangular Hollow Flange (RHF) steel beams may have openings drilled in their top surface to pass some plumbing and electrical wiring inside the RHF cavity. These openings affect the behavior of these steel beams and may reduce their resistance to bending moment. To investigate this effect, Finite Element Modeling (FEM) was used to simulate RHF steel beams with and without flange openings with several variables in geometric di mensions. The FEM results were examined using 8 experimental test specimens of RHF steel beams obtained from previous studies without flange openings. The results showed high accuracy in modeling these RHF steel beams in structural behavior and ultimate bending moment capacity. Current design codes were applied to predict the capacity of these RHF steel beams without flange openings, both from FEM results and experimental tests. The prediction values were always less than the ultimate capacity of RHF steel beams without flange openings. After verifying the results, 98 RHF steel beams with different flange opening diameters were modeled. The reduction ratios in ultimate capacity in steel beams with hollow flange openings are directly proportional to the opening diameter, hollow flange height, and steel yield stress, while they are inversely proportional to the thickness and width of the hollow flange and the steel beam web height. The reduction ratios in the ultimate capacity for RHF steel beams with flange openings are small in the compact category, and these ratios increase with the non compact and slender categories.