Psoriasis, known as the incurable disease, is a chronic inflammatory disease characterize by hyperproliferative keratinocytes. The causes of this disease are unclear, and many side effects are evident as a result of certain treatments of psoriasis. In this work, we aimed at studying the in vitro effect of each of these following Thai herbal extracts, Alpinia galanga L. (rhizome), Curcuma longa L. (rhizome) and Annona squamosa L.(leaf), containing anti-psoriatic activity on the expression of caspase 9, the protein known to play a critical role in the intrinsic apoptotic pathway. The HaCaT keratinocyte cell line was chosen as an in vitro model. Based on Western blot analysis, the extract derived from Annona squamosa L. leaf at all three concentrations tested significantly reduced the expression of procaspase 9 (P<0.05), and two higher concentrations, 0.5IC50 (3.15 μg/mL) and IC50 (6.3 μg/mL), of this herbal extract significantly increased the expression of caspase 9 protien (P<0.05). Confocal immunofluorescence microscopy subsequently confirmed this finding. Taken together, this might suggest that the ethanolic extract of Annona squamosa leaf with anti-psoriatic activity could exert its biological effect by activating the cleavage of procaspase 9 into caspase 9, thus inducing the intrinsic apoptotic pathway.