The effects of angiotensin II (Ang II) on whole-cell large conductance $Ca^{2+}$-activated $K^+$ ($BK_{Ca}$) currents was investigated in control and Apurinic/apyrimidinic endonuclease1/redox factor 1 (APE1/Ref-1)-overexpressing human umbilical vein endothelial cells (HUVECs). Ang II blocked the $BK_{Ca}$ current in a dose-dependent fashion, and this inhibition was greater in APE1/Ref-1-overexpressing HUVECs than in control HUVECs (half-inhibition values of $102.81{pm}9.54;nM$ and $11.34{pm}0.39;nM$ in control and APE1/Ref-1-overexpressing HUVECs, respectively). Pretreatment with the NADPH oxidase inhibitor diphenyleneiodonium (DPI) or knock down of NADPH oxidase (p22 phox) using siRNA increased the inhibitory effect of Ang II on the $BK_{Ca}$ currents, similar to the effect of APE1/Ref-1 overexpression. In addition, application of Ang II increased the superoxide and hydrogen peroxide levels in the control HUVECs but not in APE1/Ref-l-overexpressing HUVECs. Furthermore, direct application of hydrogen peroxide increased $BK_{Ca}$ channel activity. Finally, the inhibitory effect of Ang II on the $BK_{Ca}$ current was blocked by an antagonist of the Ang II type 1 ($AT_1$) receptor in both control and APE1/Ref-1-overexpressing HUVECs. From these results, we conclude that the inhibitory effect of Ang II on $BK_{Ca}$ channel function is NADPH oxidase-dependent and may be promoted by APE1/Ref-1.