Abstract In recent years, the support of corporate governance has attracted the attention of capital market regulatory bodies, including the stock exchange. In this regard, the corporate governance index has been taken into consideration as a monitoring tool for companies so that companies that better comply with corporate governance mechanisms can be distinguished from companies that do not comply with these mechanisms. On this basis, companies should try to improve these mechanisms and take more effective steps to create more efficiency and value and thus improve the conditions of the capital market. In this research, an attempt is made to use a new index under the title of fuzzy corporate governance index, and according to expert opinion and formulation of fuzzy inference rules, three fuzzy expert systems have been designed to evaluate corporate governance criteria. Also, the data related to 160 companies admitted to the Tehran Stock Exchange as a sample have been collected and their data has been processed using the aforementioned fuzzy systems. The output of the first system shows the quality of the ownership index and transparency in the sample companies. The output of the second system shows the quality of the board structure index and finally the output of the third system shows the quality of corporate governance in the mentioned companies. The results of these evaluations are shown in a list of the weakest and best companies in terms of ownership and transparency, the structure of the board of directors, and the quality of corporate governance.