The main method for determining the vibration characteristics of engineering constructions is modal analysis. It's a way of analyzing a system's mode shapes, natural frequencies, and damping factor. The dynamic response of cantilever beams is determined in this work with different cross-sectional shapes to find the effect of eccentricity on the dynamic response of the cantilever beam. The main goal of this research is to find and detect the natural frequencies and mode shapes of a Structural Steel cantilever beam with different eccentricities and to identify flexural or torsional natural frequencies, as well as their mode shapes that could be confused with transverse natural frequencies, and to compare the results with analytical and experimental methodologies. Results showed that torsional natural frequencies remained within the transverse natural frequency. It can be shown that, the increase of eccentricity in the cross section decreases the natural frequencies and especially the torsional natural frequencies. The results were compared experimentally and numerically using ANSYS 16.1 software. There is a strong link between the mathematical, FEA, and experimental results. The latest results can be used to calculate failure loads in a variety of situations. The mathematical application of Euler's Bernoulli's beam concept was applied. The results of the three ways have been declared satisfactory.