Earthquakes are the most important causes of structural damage that are followed with financial and life losses. Hence it is important that buildings performance be evaluated. Uncertainties in the seismic demands and structural capacity should be considered in the performance assessment of structures. Fragility curves are the current tools for performance assessment. As regards, vertical mass irregularities in building causes different behavior than ordinary structures, it is important that performance of these buildings be evaluated. This uses IDA (incremental dynamic analysis) for derivation of fragility curves. Spectral acceleration in the fundamental period and interstory drift ratio is used as IM (Intensity Measure) and DM (Damage Measure) respectively. 5, 9, 12 and 15 stories steel frames were considered. Mass irregularity was considered at top, middle and bottom story of each frame hence fragility curves develop for four cases, three cases for irregular frames and one case for regular frame. Results of investigation showedvertical mass irregularities in structures caused increase in probability of collapse. Keywords: Fragility Curve, Mass Irregularity, Incremental Dynamic Analysis, Special Steel Fram
