Traditional Chinese medicine has long been used to treat a variety of ailments including skin diseases. Our previous study has revealed the ethanolic extract of realgar, a common ingredient used in psoriasis treatment in Chinese medicine, to possess potent anti-proliferative action on cultured HaCaT cells of human keratinocyte origin. In the present study, the mechanisms of action of the observed growth inhibitory action of realgar were investigated. Several bioassay methods were employed to elucidate whether cellular apoptosis is involved in the realgar-induced growth inhibition of the skin cells. Morphologically, nuclear condensation and DNA fragmentation were observed when HaCaT cells were exposed to the realgar extract. DNA fragmentation induced by the treatment of realgar was also evident as detected by gel electrophoresis and the TUNEL method. Cell cycle analysis by propidium iodide (PI) staining demonstrated the appearance of sub-G1 peak and cell cycle arrest at the G1 phase upon realgar treatment. Quantitative analysis by annexin V-PI staining revealed that the realgar-induced apoptotic event was dose-dependent. Furthermore, realgar was able to activate caspase-3 expression when examined by Western blot analysis. Our experimental data unambiguously confirm that induction of cellular apoptosis is mainly responsible for the observed growth inhibition brought about by realgar on the HaCaT keratinocytes, and this finding helps place the traditional use of this mineral for psoriasis treatment oil a scientific footing.