This paper studies the role of social security in providing annuity insurance when there is adverse selection in the annuity market. I calculate the welfare gain from mandatory annuitization in the current U.S. social security system using a life cycle model in which individuals have private information about their mortality. I calibrate the model to the current U.S. social security replacement ratio, fraction of annuitized wealth and mortality heterogeneity in the Health and Retirement Study. The main findings of the paper are the following: 1) the overall welfare gain from having mandatory annuitization through the current U.S. social security system is 0.27 percent of consumption; 2) social security has a large effect on price of annuities because it crowds out the demand for annuities by people who have low survival expectations. This price effect has negative welfare impact of 0.29 percent of consumption. On one hand, individuals with high mortality (who will die soon and do not have demand for longevity insurance) incur large welfare losses from mandatory participation. On the other hand the effect on prices limits the benefit to the low mortality individuals. These two effects results to the overall small ex ante welfare gain.