Establishment of a social security system can be accounted for as one of the methods of poverty reduction, which is utilized by most governments. The purpose of this paper is to study Iran's Social Security System's achievement regarding poverty reduction within 1367-80 periods. The system includes insurance and supportive sections. In the insurance section, those who receive long-term supports in the forms of pensions and benefits enjoy a living standard beyond the absolute poverty line, but the amount, at constant 1369 prices, declined over the period under the study. It can be said that civil pensions have faced a similar situation. In the supportive section, poor people under the assistance of Imam Khomeini's Committee, despite the increase in real pensions, received a level of living under the absolute poverty line in the period. The application of Engle- Granger method revealed that the increase of government expenditure on social security led to a significant reduction in the percentage of households who were living under absolute poverty line.