In this paper, I investigate the optimal disability insurance (DI) when partial and fulldisability are privately observed over the life cycle. I demonstrate that in the social optimum,partially disabled agents are induced to supply labor despite substantial government transfersunless labor supply is relatively elastic and their productivity is significantly reduced. I thenapply the framework to quantitatively evaluate Korea’s DI programs, which include partialand full disability benefits. In the calibrated model, I find that welfare gains from replacingKorea’s DI programs with the corresponding optimal system amount to a 1.17% increase inconsumption. Such a reform significantly raises the utility of both types of disabled agents atrelatively small utility costs of able agents. Equity gains from this redistribution account for73.4% of the total welfare gains, whereas efficiency gains from the optimal allocationaccount for 26.6%.