Toosendanin (Tsn), a triterpenoid extracted from Melia toosendan Sieb et Zucc, possesses different pharmacological effects in human and important values in agriculture. However, liver injury has been reported when toosendanin or Melia-family plants, which contain toosendanin are applied. The mechanism by which toosendanin induces liver injury remains largely unknown. Here we reported that toosendanin induced primary rat hepatocytes death by mitochondrial dysfunction and caspase activation. Toosendanin led to decrease of mitochondrial membrane potential, fall in intracellular ATP level, release of cytochrome c to cytoplasm, activation of caspase-8, 9, and 3 and ultimately cell death. Level of reactive oxygen species (ROS) was also increased in hepatocytes after incubation with toosendanin. Catalase, the H2O2-decomposing enzyme, can prevent the reduction in ATP level and protect hepatocytes from toosendanin-induced death. The ERK1/2 (p44/42 MAP kinases) and JNK (c-Jun N-terminal kinase) were activated, but p38 MAPK was not activated by toosendanin. Inhibition of ERK1/2 activation sensitized hepatocytes to death and increased activity of caspase-9 and 3 in response to toosendanin. Inhibition of JNK attenuated toosendanin-induced cell death. These results suggested that toosendanin causes death of primary rat hepatocytes by mitochondrial dysfunction and caspase activation. Generation of ROS and MAP kinases activation might be involved in this process.