This study aims at determining the optimal environmental tax policy in the context of a dynamic model. For this purpose, clean technology diffusion was added to the AK growth model and the theoretical model has been generalized to the open economy. The main feature of the economy is creating pollution in the process of economic growth and its negative impact on social welfare. The diffusion of clean technology reduces pollution emission and has a positive effect on environmental quality and social welfare.

  The Hamiltonian solution of the model indicates that the steady state growth rate and optimal tax pollution is affected by the consumer preference toward consumption and environmental quality, pollution elasticity with respect to production, clean technology diffusion, foreign growth rate, inverse elasticity of intertemporal substitution , depreciation rate of capital and trade parameters.

  The results show that the optimal tax rate in Iranian economy is about 15 percent. Furthermore, sensitivity analysis shows that the emission elasticity of pollution subject to the production and environmental preference parameters have larger impacts on optimal tax rate than foreign growth rate and trade parameters.