The purpose of this research is to quantify the unknown factors of corporate governance caused by the tax gap and to determine its mediating effect on the relationship between the tax gap and the cost of equity. The research sample includes 149 companies accepted in the Tehran Stock Exchange covering a period of seven years from March 2013 to March 2020. The results show that there is a significant relationship between the unknown factors of corporate governance and the tax gap. Therefore, the first hypothesis is confirmed and the results of the second hypothesis test show that the tax gap variable (independent variable) is not significant on the cost of equity variable in the fourth regression related to the second hypothesis, so the variable of the unknown factors of corporate governance plays a complete mediating role. In other words, the tax gap variable can only estimate the cost of equity mediated by the unknown factors of corporate governance. First, it is expected that the results of the research can develop the theoretical foundations of previous research on the tax domain. Second, the current research can provide useful information for capital market and tax legislators. Third, it can create new ideas for conducting future research in the tax field.