Introduction: Tax as a resource of government revenue, is one of the most important devices to redistribute income and reduce the gap between the poor and the rich. Meanwhile, direct tax, exclusively income tax, is more considered than the other taxes for its simple accessibility. The importance of taxation from person income and in different viewpoints in distributive justice has led this discussion to the identification of optimal income tax features in last four decades.

Method: It is tried in this paper to study the optimal income tax by adopting the Rawlsian social welfare criterion and estimate its rate in Iran by using ARDL approach for the period 1350-85.

Findings: The findings of this study suggest that the effective optimal income tax rate in short-term (long-term) is 3.1% (1.26%) in Iran.

Discussion: It is resulted that taxation by using the estimated rates on persons' income and then redistribution to all members of society leads to the maximization of social welfare in Iran.