Abstract: Taking into account stochastic characteristics of inflow and irrigation requirement, a size-optimizing model is presented for on-farm reservoirs served as storage facilities of the irrigation district's inflow with uneven within-year distribution. First, the irrigation cycle is analyzed to determine the model's time unit for analysis. Then, the synthetic series of fountain inflow and irrigation requirement with ten-day time period are generated as the input of the proposed model. The required maximal and minimal reservoir sizes are calculated through reservoir simulation with certain operation policies in and against time sequence, respectively. The range of possible reservoir size is then discretized and each size is introduced into the model to compute its net benefits over planning time horizon by Monte Carlo method. From all possible reservoir sizes the economically optimal reservoir size is 2100 m³ per unit irrigation area, and the corresponding irrigation reliability reaches 95%.