Abstarct: The purpose of this research is to study of the factors affecting on the distribution of income by nonlinear smooth transitionregression (STR)method and for this purpose variables such as the ratio of direct taxes to total government tax revenue , real GDP per capita, the ratio of government spending to gross domestic product and growth rate of population have been used.Data used are as time series and for the period 1361-1391 .baxsed on the results of the non-linearity test, the relationship between variables has been linear and due to inappropriateness of non-linear method, ordinaryleast squares method has been used.The obtained results by using the ordinary least squares method show that government spending has increased inequality and tax policies and economic growth have led to the improvement of the income distribution. Also the effect of the growth of population on the income distribution has not beensignificant. JEL classification: C01، C23، D31