Abstract: Ecological flow is a key factor in reservoir ecological operation to take into account the annual variations of runoff, but its inter-annual variations are also important. This paper presents an optimal operation model for maximizing the power generating capacity of reservoirs under the constraint of ecological flow in a case study of the Han to Wei River water diversion project. In this model, the standardized precipitation index is used to divide long runoff series into typical series of different levels, and stochastic flow duration curves are used for estimation of the ecological flows of the typical series. Calculations reveal that after years of reservoir operation in the schemes optimized using the model, the average generating capacity is 514 and 505 million kW?h under the minimum and appropriate ecological flows, respectively, which agrees with the value of 503 million kW?h in the project planning. This study shows that distinguishing different typical annual runoff series in optimizing reservoir operation can reflect the effects of inter-annual variations in the river runoffs so that the optimal operation scheme for every typical year can basically meet the ecological flow demand. However, in the years of extremely low flows, it does not meet the ecological demands, which needs special attention.