Euonymus sieboldianus is a dioecious and deciduous shrub growing in Japanese Archipelago and southern Korea. We asked following three questions: (i) Does flower number differ between sexes? (ii) Does flower size differ between sexes? (iii) Does sex ratio differ with light intensity that a plant receives? We chose 271 individuals of E. sieboldianus in 5 plots in Obihiro, Eastern Hokkaido, Japan. Light intensity that plants receive is classified into five classes (increasing order from 1 to 5) by eyes. The number of flowers increased with the class of light intensity, and basal diameter. It was larger in male than in female when the effects of light intensity and basal diameter are canceled. Flower size was larger for male ( 89.2 ± 11.4 mg ) than for female ( 48.3 ± 4.1 mg ). Those results show that reproductive effort is larger for male than for female at flowering stage. Female ratio (female / (female + male ) ) in the number of individuals increased from 0.5 for those plants with the lowest class of light intensity to more than 0.8 for those plants with the highest class of it. This result shows that the favorable conditions to grow favor female to male, though we did not make clear the reasons why female is favored under strong light conditions.