Nowaday the amount of water resource to generate the hydro-power energy has decreased as that of the water supply has increased. In case that the national market of the energy will be in free competition, the energy producer need to suggest the amount of the optimal supply with the hydrological reliability for a deal. In this study the optimal reservoir operation was performed by the linear programming and the optimal reliabilities of inflows and the power supply were obtained by the one dimensional search technique to estimate the energy with the optimal inflow reliability which the power system of the Yongdam multipurpose dam in Geum river can produce. And the main results were presented.