Apoptosis signal-regulating kinase 1 (Ask1) is one of mitogen-activated protein kinase kinase kinase (MAPKKK) for cell differentiation and apoptosis. The aim of the present study is to evaluate whether RNA interference against Ask1 (Ask1-siRNA) down-regulates the expression of Ask1 and prevents apoptotic neuronal cell death after ischemia/reperfusion (I/R) in mice. Mice were subjected to intraluminal suture occlusion of the middle cerebral artery for 1h, followed by reperfusion. The Ask1-siRNA or a control-siRNA was introduced using osmotic pump intracerebroventricularly at 3days before I/R. The expression and mRNA of Ask1 were evaluated by Western blot and RT-PCR after I/R with time. Immunohistochemistry and TUNEL assay were also investigated to evaluate the effect of Ask1 on cerebral infarction by Ask1-siRNA treatment. The expression of Ask1 was increased significantly at 8h after I/R. The level of mRNA and protein of Ask1 down-regulated after treatment of Ask1-siRNA and subsequently cerebral infarction volume was reduced. Our results suggest the increased Ask1 expression induce apoptotic cell death after I/R. And we also demonstrated that Ask1-siRNA attenuates upregulation of Ask1, which was followed by the reduction of infarction in ischemic brain after I/R. Ask1-siRNA could represent a molecular target for prevention of ischemic stroke.