Analyzing the vibrations of beam components is highly beneficial for engineering analysis and design. Modal analysis is an investigation used to determine modal parameters such as natural frequency, mode shape, and damping ratio of a system. This paper presents findings of modal analysis of reinforced concrete beams with varying thicknesses using theoretical and numerical methods. Three beams of thickness 200 mm, 250 mm and 300 mm were analyzed with constant width and length of 200 mm and 1500 mm. The aim is to investigate the influence of thickness variation on the natural frequencies of the beams. The findings between the two approaches were compared to determine the prediction accuracy. The findings of this study proved that the increasing thickness of the beam has significant impact on the modal frequencies’ computations using the two approaches. There is a steady increase in percentage errors on the numerically analyzed frequencies when compared with theoretical frequencies, as the thickness increases. From the results, it was also found that specific increment of beam thickness can pose limitation in the modal analysis procedure using numerical computation. Finally, the study also demonstrated the significant differences between the beams modelled with and without rebar, verifying the effect of steel reinforcement in dynamic behavior of the beam.