Cadmium (Cd) is absorbed readily by rice plants, and is transferred to humans when contaminated rice is consumed. The addition of selenium (Se) to the nutrient solutions has been shown to reduce the accumulation of Cd in rice seedlings. However, as the relevant underlying mechanism remains unclear. Hydroponic experiments were performed in this study to investigate the effects of selenite or selenate on the Cd subcellular distribution and xylem transport in rice seedlings (Oryza sativa L.) under Cd stress. The antioxidative defense responses in rice plants were also assayed. Results showed that the supplementation of both Se forms decreased the Cd accumulations in rice roots and shoots. Selenite addition significantly decreased the Cd contents in different subcellular fractions of rice roots, while increased the proportion of Cd distributed to soluble cytosol by 23.41%, and decreased the Cd distribution in organelle by 28.74% by contrast to the Cd-alone treatment. As for selenate addition, only the Cd distribution ratio of cytosol was increased by 13.07%. Furthermore, 55.86% decrease of Cd concentration in xylem sap was displayed when adding selenite, while little change was found in rice plants exposed to the treatment co-applied with selenate. Hydrogen peroxide (H₂O₂) and malondialdehyde(MDA) contents in rice roots were elevated under Cd stress, and the addition of selenite and selenate decreased H2O2 levels by 77.78% and 59.26%, respectively. Cd and Se co-exposure elevated the glutathion (GSH) accumulations in rice shoot and roots, and the degree of the increase was: co-applied with selenite > co-applied with selenate > Cd alone treatment. The exposure of Cd significantly increased the catalase (CAT) activity in roots, while decreased that in shoot. Upon selenite or selenate supplementation, CAT activity in rice root was increased compared to that when Cd was applied alone. Cadmium or Se addition did not considerably change the activities of peroxidase (POD) or ascorbate peroxidase (APX) compared to the control.