The effect of lanthanum (Ⅲ) on reactive oxygen metabolism of soybean seedlings under elevated ultraviolet-B radiation (UV-B: 280～320 nm) at 0.15 and 0.45 W·cm-2 levels respectively was studied through hydroponics in the laboratory. Plasmolemma permeability and contents of malonadialdehyde (MDA), hydrogen peroxide (H2O2), and proline gradually increased during the imposition of UV-B radiation and subsequently decreased during recovery from UV-B stress. The dynamic tendency of catalase (CAT) activity was similar to that of the above four indices. The activity of peroxidase (POD) initially increased, then remained at a high level, and finally dropped steeply when soybean seedlings were exposed to a low dosage of UV-B radiation. However, POD activity rose throughout and declined slightly on the eleventh day when soybean seedlings were stressed by a high dosage. With the addition of La(Ⅲ) of 20 mg·L-1, the rising tendency of plasmolemma permeability and contents of MDA, H2O2, and proline were slowed down during the stress period, whereas the declining speed was accelerated during the recovery period. The activities of CAT and POD were higher than those without La(Ⅲ) in all experiments. It suggested that the regulative effect of La(Ⅲ) on antioxidant enzymes such as CAT and POD could strengthen their capacities to scavenge reactive oxygen species (ROS), decrease contents of MDA and proline, and maintain normal plasmolemma permeability. Further more, the protective potential of La(Ⅲ) was better under low UV-B radiation than under a high one.