For a basin with existing reservoirs, the necessity of additional water demands has been proposed, as well as a reevaluation of existing reservoir yield has been proposed. The objective of this study is to reevaluate a multipurpose reservoir yield and to assess the possibility of additional water supply according to increase of downstream water demands. Andong and Imha Reservoirs are selected for reevaluation. The standard reservoir operation rule model and the HEC-ResSim model were used for reservoir simulation for 30 years (1979~2008). In this study, water supply reliability was set up as 96.7% and 95.0% with yearly and monthly evaluating unit. In case of 95% water supply reliability with yearly evaluating unit, water supply capability of Andong reservoir was evaluated as 893MCM and water supply capability of Imha reservoir was evaluated as 382MCM, and that results showed that water yields for both reservoirs are less than the original designed yields.