Abstract: To develop further insight into the effect of rare-earth on water metabolism in plants under UV-B radiation stress, the effect of Ce(Ш) on stomatal density, stomatal conductance (Gs), abscisic acid (ABA) content, hydrogen peroxide (H₂O₂) content, and catalase (CAT) activity in soybean (Glycine max) seedling leaf exposed to ultraviolet-B radiation (UV-B, 280 ~ 320 nm) was analyzed in hydroponics culture under laboratory conditions. Static experimental results show that 20 mg·L^-1 CeCl₃ increases stomatal density of paraxial leaf back surface compared with the control, whereas the reverse effect occurs in soybean seedlings exposed to high UV-B radiation (0.45 W·m^-2). Ce(Ш) plus UV-B treatment retards decrease in stomatal density. On the other hand, dynamic experimental results show that 20 mg·L^-1 Ce(Ш) increases Gs compared with the control. UV-B radiation induces a decrease in Gs. Also soybean seedling Gs under Ce(Ш) plus UV-B treatment is higher than in UV-B treatment alone. This shows that Ce(Ш) retards decrease in soybean seedling Gs under UV-B treatment and induces a good recovery at the end of the experiment. In comparison with the control, UV-B radiation induces ABA and H₂O₂ accumulation while increasing CAT activity. However, Ce(Ш) reduces the above-mentioned accumulations while increasing CAT activity in soybean seedling leaf under UV-B radiation stress compared with UV-B treatment alone. The experimental results reveal that Ce(Ш) reduces damage from UV-B radiation stress on water metabolism in soybean seedling.