To determine the sex-expression of Juglans ailanthifolia, we monitored its flowering phenology for 3 years in two natural populations and observed the following four mating types: protogyny, protandry, female and male. In each population, the number of protogynous trees was most numerous, followed by protandrous tree. These monoecious types constituted 61–95% of the individuals at each site in each year. Observations of flowering phenology of the monoecious types showed that female and male functions were temporally segregated within individual trees and that the sexual functions of each protogynous and protandrous mating type were synchronous and reciprocal. Such a system may help to reduce selfing and promote outcrossing. Trunk size was smaller in the unisexual types than in the monoecious types. Sixty-seven percent of trunks were oblique due to snow pressure in the unisexual types, whereas only 23% were oblique in the monoecious types. It seemed that unisexuality is a temporary trait because the changes in mating category occurred mainly from unisexual to monoecious types and inverse changes were very few.