This study was designed to elucidate the synergistic cytotoxic mechanisms of the cotreatment of Paljintanghabhwajeoghwan (Paljin) and As₂O₃ in human lung cancer cell line, H-460. The combination of Paljin and As₂O₃ synergistically augmented the cytotoxicity of Paljin and As₂O₃ in H-460 cells. The nature of cytotoxicity was revealed as apoptosis which characterized by chromatin condensation and fragmentation in DAPI staining. Mitochodrial membrane potential transition was observed in H-460 cells treated with Paljin and As₂O₃. The apoptotic cytotoxicity of Paljin and As₂O₃ was accompanied by the cleavage of PARP and ICAD. Of note, pro-apoptotic Bak protein was obviously increased. However, the expression of p53 was not affected by the cotreatment of Paljin and As₂O₃. In addition, the expression of DR5 was increased by the cotreatment of Paljin and AS203. This results suggest that the synergistic cytotoxicity of the cotreatment of Paljin and As₂O₃ might be caused by the changes of the expression levels of a lots proteins, such as PARP, ICAD, Bak, DR5, which play pivotal roles in survival or death of cells.