In structural buildings, mass irregularity is considered to exist where the effective mass of any floor is more than 150% of the effective mass of an adjacent floor. Mass irregularity is an important factor affecting structural responses under seismic loadings. In the conventional methods, the buildings are designed in accordance with the structural codes on the basis of static or dynamic linear analysis. When such buildings (especially irregular buildings) are located in a high seismic zone, some structural elements experience plastic deformations. Therefore, the structural engineer needs to have a thorough understanding of the nonlinear dynamic response of these structures. In this paper, nonlinear time history analyses are carried out to study the nonlinear response of 5, 10 and 15 storey buildings with vertical mass irregularity. The moment resisting frames are designed based on Iranian building code. The mass irregularity is created by increasing the effective mass (weight) of some floors relative to other floors (2 or 3 time). These floors with additional masses can be located in the first floor, middle floors or upper floors of the building. The storey drift ratio, plastic hinge rotation, hysteretic behavior, force distribution of stories and stress in frame elements are investigated. The results show that the storey drift ratio increases when vertical mass irregularity occurs. Also the seismic response of building with mass irregularity is varying relative to location and amount of irregularity and it is dependent on the earthquake record selection. Furthermore, buildings with vertical mass irregularity usually exhibit lower performance than the regular buildings. Then the largest seismic demand is found for the structure with mass irregularity.