In this paper, the seismic performance of irregular steel structure with varying irregularity ratio has been investigated. For the study purpose, two different models with vertical geometric irregularity and three different models with plan irregularity according to IS 1893 (part 1)-2002 have been considered. The irregularity ratio, A/L (where A is offset, and L is base width), has been varied from 0.2 to 0.8. Irregular structures have been modelled using ETABS, a finite element software, and plastic hinges are assigned to incorporate the inelastic seismic behaviour of structures. Performance of thirteen irregular structures has been compared with regular bare frame structure in terms of base shear carrying capacity, roof displacement and performance point, using pushover analysis. Two slab models with shell element and membrane element considered for all structural models. The results indicate that as irregularity ratio increases, base shear carrying capacity and performance point of irregular structure decrease. Structural models considered slab as shell obtained higher seismic performance compared to slab as membrane.