"Social discount rate" (SDR) is one of the most important macro variables for policy making that has a crucial effect on the other macro variables such as growth and sustainability and ethical variable such as intergenerational justice. The main problem of this research is the analysis of the SDR effects on the sustainability and intergenerational justice and then determining the optimum SDR. The realization of the "sustainability criterion" requires that the social discount rate be zero in order to prevent the decline of the optimal consumption path. Also, according to the ethical approach, "intergenerational justice" requires equal treatment with current and future generations, and it is ethically inappropriate to place less importance on future generations and to confiscate and consume the resources they are entitled to now. By adopting this ethical approach in this paper, a "zero percent social discount rate" is prescribed, which will lead to more investment and future welfare.