The purpose of this research is to investigate the reasons for Tax evasion in companies, which uses two independent variables (financial constraints and Tax Planning opportunities) and tow dependent variable (firm value and Tax evasion by Tax difference method) in the form of 13 models. The 11 indicators have been considered for the variable of financial constraints of companies, and the model is implemented for all these indicators. The research was conducted in the 5-year period from 2015 to 2019 in the Tehran Stock Exchange, and Eviews software was used to analyse the data and fit them for 3 research hypotheses. The results of the research show that the Opportunity for Tax Planning has a negative effect on the value of the company, and the increase in the Opportunity for Tax Planning and subsequently Tax evasion causes a decrease in the value of the company. Also, the research results showed that there is a significant relationship between Tax Planning Opportunity and Tax evasion (by Tax differences method) of companies, while there is no positive relationship between financial constraints and Tax evasion (by Tax difference method) in companies that have Tax Planning opportunities.