Many tree species have annual fluctuations in flowering and seed production. However, there are few long-term, quantitative data for dioecious tree species. Fraxinus platypoda Oliv. is a riparian tree that reaches up to 40 m in height, growing in the cool-temperate zone in Japan. This study examined the annual fluctuation in its flowering over the long-term and whether the flowering is synchronized between females and males. We checked the flowering of 60 individuals from 1990 to 2014 using binoculars. The flowering degree was divided into five ranks. Clear annual fluctuations in flowering occurred in female and male trees over the 25 years. Male trees flowered more with less fluctuation than females. Synchronous flowering occurred in females and males until 2001, while the relation was unclear in recent years. After 2001, the flowering patterns clearly changed, with male trees undergoing mast flowering every year, while the flowering cycle of female trees became shorter than before 2001. There was no significant correlation between the weather conditions and annual fluctuation in flowering degree. These facts suggest that annual fluctuation in the flowering degree can occur due to the resource balance of each tree without any weather fluctuations. The change in fluctuation after 2001 might be a result of climate change.