Selenium, an essential trace element, showed the significant protective effects against liver and kidney damage induced by some heavy metals. However, the mechanism how selenium suppresses cadmium (Cd)-induced cytotoxicity remains unclear. In this study, we investigated the protective mechanism of selenium on Cd-induced apoptosis in LLC-PK(1) cells via reactive oxygen species (ROS) and mitochondria linked signal pathway. Studies of PI and Annexin V dual staining analysis demonstrated that 20 microM Cd-induced apoptosis as early as 18 h. A concomitant by the generation of ROS, the loss of mitochondrial membrane potential, cytochrome c (cyt c) release, activation of caspase-9, -3 and regulation of Bcl-2 and Bax were observed. N-acetylcysteine (NAC, 500 microM), a free radical scavenger, was used to determine the involvement of ROS in Cd-induced apoptosis. During the process, selenium played the same role as NAC. The anti-apoptosis exerted by selenium involved the blocking of Cd-induced ROS generation, the inhibition of Cd-induced mitochondrial membrane potential collapse, the prevention of cyt c release, subsequent inhibition of caspase activation and the changed level of Bcl-2 and Bax. Taken together, we concluded that Cd-induced apoptosis was mediated by oxidative stress and selenium produced a significant protection against Cd-induced apoptosis in LLC-PK(1) via ameliorating the mitochondrial dysfunction.