The vegetative growths in some tree species are reduced in mass reproduction years. Because the main timing of trunk radial growth and seed ripening is separated in Fagus crenata, two hypotheses are considered; 1): reduction of growth rate due to resource allocation both to vegetative growth and reproduction, and 2): bringing forward the radial growth saturation due to the switching of the resource allocation from vegetative growth to reproduction. To evaluate the effect of masting on the growth phenology of F. crenata, high-resolution patterns of radial growth in F. crenata during a non-reproduction (2014) and a heavy-reproduction year (2015) were monitored by using digital dendro-meters. Trunk radial growth in each tree started synchronously just after leaf flushing in both years, but was slightly early in 2015. Compared to non-reproduction trees, reproduction trees exhibited lower annual radial growth and growth rate in 2015, but had similar period of radial growth. These results suggest that the decrease in trunk radial growth of F. crenata in masting year was resulted from the reduction of growth rate.