Steel frames are very ef¿cient in resisting forces generated from ground motion because of their ability to dissipate seismic energy through large inelastic deformation and damping. Therefore steel is often used for buildings in seismically active regions. Furthermore nowadays construction of light buildings is common, so it is important to study the behaviour of high-raised building under self-weight and dynamic loads. The design of steel structures subjected to natural hazards such as earthquakes demands safety of structures which is governed by the fundamental frequency and the magnitude of frame response. Reasonably accurate assessment of these characteristics is an essential part of design and analysis processes. Also the effect of frame height as well as number of stories has been considered Nowadays, the trend in using castellated sections in steel frames is increasing since using castellated I-beams makes the building lighter and more economical. By considering the importance of periods of building it is necessary to study the effect of openings on natural frequencies of steel frames.