The Demand for Open Source Software (OSS) is increasing day by day. However, its end users still face challenges using such software. Therefore, this study is conducted to propose a fuzzy usability model to be an approach for evaluating the usability of the open source software. In order to propose such model, six common usability characteristics have been considered, namely: Learnability, understandability, attractiveness, operability, efficiency, and memorability. Some of these characteristics are related to the nopen source software's features (Learnability, usability, attractiveness, operability); and the rest are related to the end users. Finally, the Matalab Simulink software (Fuzzy Logic Toolbox) has been employed to simulate and to validate the proposed approach model.