Masting is an intermittent synchronous production of large seed crops by plant populations. The cause and mechanism of masting were analysed using a resource budget model for an individual plant. The model produced characteristic masting behavior that is made up of intermittent large crops, crop failures and semi-regular masting cycles. The ratio of seedling cost to flowering cost (RC ) played the most important role in the model. The species with a large RC value (RC e 1) showed intermittent fruiting whereas species with a small value (RC Q 1) showed constant fruiting. With increasing RC values, the fruiting interval became longer, and the occurrence became more unpredictable. When the model ignored the dependence of fruiting efficiency on the ratio of flowering individuals, flowering and fruiting did not synchronize among individuals in a population. In contrast, when the model included the dependence of fruiting efficiency on the ratio of flowering individuals, synchronization of flowering and fruiting within a population occurred. The synchronization was also dependent on the kind of pollination system. The present model suggests that masting can take place due to the resource balance of each plant even without any interannual environmental fluctuations, and may result in evolutionary benefits to each individual.