In-plan irregularity can lead to severe damages of RC buildings as a result of additional stresses on the RC elements due to torsion effects. Considering the effect of the in-plan irregularities becomes a challenge for the seismic performance assessment. In this paper, 3D numerical models of four- and six-storey RC buildings with different levels of eccentricity were developed to estimate the behaviour of in-plan irregular RC buildings. Instead of changing the dimensions of the section of RC columns or using different structural configurations to impose eccentricity, the mass was lumped in a certain point which works as an artificial centre of mass (CM). Then, the artificial centre was controlled to produce several levels of irregularity. Nonlinear static pushover analysis was used to assess the performance of the considered building due to the lateral loading. The lateral capacity of the studied buildings were plotted, as such, it has been observed that the lateral capacity decreases with the increase of the eccentricity ratio due to the increase of the level of torsional stresses in those with higher eccentricity ratios. In addition, the eccentricity in the direction, which has a lower stiffness, is more effective on the building strength than the eccentricity in other direction.