The intermittent and synchronized production of large numbers of seeds or flowers by a population of plants is well known as mast seeding, or masting. Several hypotheses have been proposed to explain the mechanism of masting. It is most likely that plants respond to both internal resource level and external environmental cues to initiate flowering. However, how and in what processes these factors influence the floral transition is unknown because direct observation of development of floral organs at initial stage is impossible. In order to explore the effect of these factors on flower development, we studied the relationship between the proximate factors and expression level of floral meristem identity gene; LEAFY homolog (FcLFY) using mature Fagus crenata for consecutive 2 years in natura. We measured seasonal pattern of FcLFY expression in bud using real-time quantitative PCR, and analyzed the relationship between FcLFY expression and process of floral bud differentiation. Sizes of floral organs and the level of FcLFY expression showed positive correlation, meaning that FcLFY expression can be used as a marker to visualize the process of flower development. We further analyzed the relationship between concentrations of sugar and nitrogen in branches and the level of FcLFY expression. We discuss how this method can be applied for in-depth understanding of the mechanism of masting.