ANSYS 10.0, a finite element software, has been used to simulate simply supported RC beams consisting of circular openings with varying diameters in 3-D nonlinear finite element method. The RC beam model is created using SOLID65, SOLID45 and LINK8 elements representing both linear and non-linear behavior of concrete, steel plate and reinforcing bar respectively. The inherent assumption is that there is full displacement compatibility between the reinforcement and the concrete and no bond slippage occurs. The FE beam model is verified against experimental test data of RC solid beam without opening available in literature and then a number of verified models of simply supported RC beams with circular opening are loaded monotonically with two incremental concentrated loads. A model with equivalent square opening of 133 mm in width is also analyzed. An attempt is made to know the effects of circular opening size on the behavior of RC beams from load-deflection curve, crack pattern and stress distribution. RC rectangular beams with circular openings of diameter less than 44% of the depth of beam (D) has no effect on the ultimate load capacity but circular openings with diameter more than 44% of D reduces the ultimate load capacity at least 34.29%.