Objectives : The purpose of this study was to identify the anti-tumor effects of realgar on various cancer cells through molecular biologic and cellular biologic methods. Materials & Methods : We used 5 kinds of cancer cell lines:stomach cancer cell (AGS), glioma cells (T98G, A172, SNU-489) and prostate cancer cells (LNCaP). We injected the boiled extract of realgar. $50{mu}$g/ml and $100{mu}$g/ml to culture media (ml) for 24 hours. We examined the morphological changes under an inverted microscope and a fluorescence microscope. We measured the suppressive effect on viability of 5 kinds of cancer cells via XTT assay. We examined the effect on the revelation of PARP cleavage, Bcl-2 protein and Bax protein by western blot analysis. Results : The extract of realgar caused markedly morphological changes on AGS, T98G, SNU-489, and LNCaP. All of them showed withdrawn and floating appearance. The suppressive effect on viability of AGS, T98G, A172, SNU-489, and LNCaP showed that each test group had more suppressive effect on viability of AGS, T98G, A172, SNU-489, and LNCaP than the control group, which was statistically significantly (p<0.01). The extract of realgar did not induce PARP cleavage in AGS, T98G, A 172, SNU-489, or LNCaP. In the revelation of protein related to apoptosis, the protein levels of Bcl-2 decreased and the protein levels of Bax increased in AGS, T98G, SNU-489, and LNCaP treated with realgar. The protein levels of Bcl-2 decreased and the protein levels of Bax did not change in A172 treated with realgar. Conclusions : This experiment showed that realgar has anti-tumor effect on stomach cancer cells (AGS), glioma cells (T98G, SNU-489L and prostate cancer cells (LNCaP)