Multi-story steel structures may be used in industrial or residential buildings. The floors are not always of the same height. The ground floor used to have higher height in many occasions; while other floors may have different heights due to architectural or mechanical installation requirements. The nternational codes of practice do not allow the equivalent lateral force (ELF) procedure to be used for structures with vertical irregularities. This means, a detailed analyses shall be carried out each time a structure has a different floor height or different floor mass. The purpose of this study is to investigate the definition of irregular structures for some vertical irregularities: stiffness and mass. About 300 steel frames representing different situations of four buildings with various inter-story heights and mass ratios are considered for a detailed parametric study. The lateral force-resisting systems (LFRS) considered are steel moment resisting frames (SMRF) with both rigid and semi-rigid connections. The effect of vertical irregularity on the dynamic characteristics of five-, six-, eight-, and ten-story steel frames is investigated. The effect of higher modes (up to the fourth mode of vibration) is considered in the study. The results show that the rigid and semi-rigid connections had the same trend of effect with the change of the floor height or mass with negligible difference. The first mode of vibration is found to be highly affected by the change of the height of the lower third or the mass of upper floors of the building. The change in upper floors heights resulted in a negligible effect on the first mode of vibration, while it has the dominant effect on the higher modes of vibrations. The change of the height or mass of floors at position of the inflection points of the higher modes is found to result in a minor change of the dynamic characteristics of the structure.