Objectives : We are aimed to identify anti-tumor effects of realgar on some kinds of cancer cells through molecular biologic methods. Materials & Methods : We used 5 kinds of cancer cell lines: lung cancer cells(A549). stomach cancer cells(KATO) and neuroglioma cells(SUN-1118. U-87MG, U-373MG). We injected the boiled extracts of realgar $50{mu}g$. $100{mu}g$ to cultural media( ml )for 24 hours. We measured the killing effects on 5 kinds of cancer cells through inverted and fluorescence microscope, the suppressive effects on viability of those cells via XTT assay and the effects on the revelation of Bax and Bcl-2 proteins related to apoptosis by western blotting. Results : In the changes of morphology, the extracts of realgar showed more significant killing effects on all cancer cells. especially KATO, SNU-1118, U-87MG, U-373MG, than the control group with dose dependence, which was statistically significant. In XTT assay, the extracts of realgar showed more suppressive effects on viability of all cancer cells, especially KATO and U-373MG, than the control group with dose dependence, which was statistically significant. In the revelation of proteins related to apoptosis, the extracts of realgar increased the level of Bax and decreased that of Bcl-2 in all cancer cells with dose dependence. Conclusions : We identified that realgar had more anti-tumor effects on stomach cancer and neuroglioma than on lung cancer in the experiments above. However, these basic experiments were performed in vitro. We hope the anti-tumor effects of realgar will be practically identified through more progressive research.