Abstarct: Due to population growth and migration to the cities, industrial technologies in construction have been more popular in recent years. One of these methods is tunnel form concrete building structures. Although most of the structures with the Tunnel Form Concrete system are regular structures (in plan or height) it is necessary to consider the mass irregularity on structural behavior in this thesis. Firstly, six structures with or without mass irregularity were modeled and designed with ETABS2016 and then, the structures were modeled with PERFORM3D. After the verification of the finite element model, the seismic assessment of structures was performed by time history analysis and then IDA analysis. Fragility curves were obtained using performance levels according to the HAZUS guideline. Seismic modification factor, overstrength factor, ductility ratio, and median fragility were obtained and discussed during this thesis. Finally, the suitable modification factor in tunnel form concrete system considering the mass irregularity effect was proposed using the analysis results. The results showed that the irregularity caused a lower seismic modification factor and lower ductility ratio. Moreover, by increasing the height of structures in both regular or irregular form, the seismic modification factor and ductility ratio increased due to a higher structural period (T).