In the present paper, dynamic characteristics of castellated beams are investigated and compared with the plain-webbed ones. Firstly, using the FEM-based software package ANSYS, modal analyses are performed on plain-webbed and castellated simply supported beams. The mode shapes are extracted and discussed. Afterwards, during a frequency-domain analysis, a white noise dynamic load with an appropriate frequency range is applied to both plain-webbed and castellated beams and their dynamic behaviors are studied through extracting the power spectral densities (PSDs) of various structural responses. The effect of full and local lateral bracing of the beams is considered in the present study. It can be concluded that the loading pattern is a critical factor in investigating the dynamic properties of simply supported castellated beams. For example, when a uniformly distributed load having white noise characteristics is applied to the whole length of the beam, some mode shapes can not be detected. However, when the same load is exerted on the half of the beam length, all mode shapes appear in the PSD of the acceleration response.