This study was designed to elucidate the synergistic cytotoxic mechanisms of the cotreatment of Gagamsibjeondaebo-tang (GSD) and AS₂O₃ in human lung cancer cell line, H-460. The combination of GSD and AS₂O₃ synergistically augmented the cytotoxicity of GSD and AS₂O₃ in H-460 cells. The nature of cytotoxicity was revealed as apoptosis which was characterized by chromatin condensation and fragmentaton in DAPI staining. The apoptotic cytotoxicity of GSD and AS₂O₃ was accompanied by the cleavage of PARP. Of note, the expression of pro-apoptotic BclXS protein was increased, but the expressions of Sax and BclXL was not affected in H-460 cells treated with GSD and AS₂O₃. In addition, antioxidant NAC completely blocked the apoptotic death of H-460 cells by GSD and AS₂O₃. In conclusion, this results suggest that the cotreatment of GSD and AS203 induces the synergistic apoptotis of human large cell lung cancer cell line, H-460 via the induction of BclXS and reactive oxygen species (ROS).