The deterministic seismic assessment methods remain insufficient to define structural behavior under earthquake effect due to the uncertain and random variables in the analysis. Including the probabilistic approaches into these analyses for definition of seismic structural behavior will give more rational results. In this present study, it is aimed to realize the probability based seismic analyses of reinforced concrete frame structures and to emphasize that a probabilistic seismic assessment is necessary to define the structural seismic behavior realistically. For this aim, first, nonlinear analyses as push over analysis and time history analysis with selected earthquake data are realized. The results of push over analyses are compared with time history analyses results. Second, for the same R/C frame structures, the fragility analysis which is a system reliability analysis with correlated demands and capacity is performed to establish the probabilistic characterization of the demands of the R/C frame structures. Fragility analyses are realized with three different parameters as (peak ground acceleration), PGV (peak ground velocity) and PGD (Peak Ground Displacement) based. The obtained results are compared to evaluate the parameters used in fragility analysis.