Abstract: Seismic ductility reduction factor was investigated in this paper for vertically irregular structures subjected to pulse-like ground motions. It was expressed as the production of seismic ductility reduction factors for vertically regular structures and a modification factor. Based on multi-degree-of-freedom (MDOF) lumped mass shear type models, the method to determine the modification factor was introduced, and its influencing factors were investigated. The results show that ductility reduction factors for vertically irregular MDOF systems are clearly smaller than those for vertically regular MDOF systems. The modification factor mainly depends on irregularity ratio and ductility ratio; the smaller the irregularity ratio or the larger the ductility ratio, the smaller the modification factor; while the influence of storey number and fundamental period is small. In addition, the modification factors for structures under velocity pulse-like ground motions are less than those under non-pulse-like ground motions. At last, the average results were suggested as modification factors, which were compared with the values specified in European seismic code.