Cerebral vascular dysfunction and associated vascular complications often develop over time in type-2 diabetes, but the underlying mechanisms are not wholly understood. The aim of the present study was to investigate whether large-conductance Ca2+-activated K+ (BKCa) channels in cerebral artery smooth muscle cells (CASMCs) were impaired in experimental model of type-2 diabetes, and the changes could account for cerebral vascular complication in type-2 diabetes. Sprague-Dawley rats were fed with high fat and glucose diet for 8 weeks and then injected with streptozotocin (STZ/30 mg/kg i.p.). Three months after injection of STZ, the alterations of BKCa channels were assessed by using multi-myograph system, patch-clamp, RT-PCR and Western blot. Our results show that the model is characterized by insulin resistance, hyperglycaemia, hyperlipidemia and moderate hypertension, which resembles the clinical manifestation of patients with type-2 diabetes. Inhibition of BKCa channels with 1 mM tetraethylammonium (TEA) or 1 μM paxilline (PAX) causes smaller constriction in type-2 diabetic cerebral basilar arteries than control arteries. The contractile efficacy of 5-Hydroxytryptamine (5-HT) is substantially reduced by TEA or PAX pretreatment in control > diabetic basilar artery rings. The response to 5-HT in diabetic basilar artery rings is higher than that of control artery rings after activation of BKCa channels with NS1619. The whole-cell K+ currents are significantly decreased in type-2 diabetic CASMCs compared to control, and the sensitivity of BKCa channels to voltage, the specific inhibitor and opener are all diminished in diabetic CASMCs. The expression of BKCa channel β1, but not α-subunits is markedly reduced at both of mRNA and protein levels in endothelial-denudated cerebral arteries. In conclusion, type-2 diabetes downregulates BKCa channel β1-subunits in CASMCs, resulting in reduced activity of BKCa channel, increased vascular tone and blood pressure, thereby contributing to cerebral vascular complication in type-2 diabetes.