The performance assessment is usually carried out by using plastic hinge rotations at the ends of beam and column sections in the nonlinear structural analysis. The guidelines and requirements are given in some recommended documents (FEMA356, ATC40) and in Turkish Earthquake Resistant Design Code 2007 (TERDC 2007). The rules and the boundaries in various recommended documents and codes are defined within strict intervals, i.e. crisply. Approximations in structural system modeling and numerical analysis make the accuracy of those limitations questionable. This paper deals with an efficient approach for the performance evaluation of existing structural systems. It is shown that the performance levels can be flexible by using certain weighted values depending on the number and deformation levels of the elements (flexible performance). Herein, fuzzy set theory is used with fuzzy inputs, a set of corresponding rules (rule base) and a defuzzification of the outputs for the performance level of structural systems. In the numerical treatment of the method, the damages and the corresponding performance levels in FEMA356 are considered and MATLAB Fuzzy Toolbox and Simulink software options are used. The methodology can be tested by use of real data (obtained from non-linear analysis of structural systems) or generated data. The method is tested with a frame system building and a bridge pier data and the necessary performance levels are determined and finally the results are discussed comparatively in tables and graphs.