It is known that leaf flush timing differs among the twigs in a tree crown. Do the stages of leaf phenology have any relationship to twig vessel formation? We observed the stages of vessel formation at different stages of leaf phenology. Beginning in early spring, shoot and stem samples were picked twice a week from 5 diffuse-porous Fagus crenata and 5 ring-porous Quercus acutissima trees in the Kamigamo investigational forest (Field Science and Research Center, FSERC; Kyoto University, Japan). We found that the progression of twig vessel formation was relative to both leaf development and time. Leaf formation preceded twig vessel formation in diffuse-porous Fagus crenata, whereas leaf formation occurred simultaneously with vessel formation in ring-porous Quercus acutissima. In addition, the time-lag in vessel formation between the twigs and stems in diffuse-porous Fagus crenata was longer than in ring-porous Quercus acutissima. Our findings suggest that there is a relationship between the stages of leaf phenology and twig vessel formation; the timing of vessel formation and leaf flush differs among the twigs in a tree crown, and each species has its own order of priority for vessel and leaf formation.