The balance of catalyst loading, activity and stability remains a challenge for the anode of proton exchange membrane (PEM) water electrolyzers. Here we report a nano-size IrOx/Nafion(center dot) composite catalyst that exhibits both outstanding activity for oxygen evolution reaction (OER) and stability in a PEM water electrolyzer. The IrOx/Nafion(center dot) catalyst layer is fabricated using a flame-based cost-effect process, reactive spray deposition technology. The IrOx/Nafion(center dot) catalyst shows > 10 times improvement in OER mass activity compared to IrOx nanoparticles synthesized using the wet chemistry method. The IrOx/Nafion(center dot) catalyst also achieved similar to 4,500 h of stable operation in MEA electrolyzer at 1.8 A cm(-2) and 80 degrees C with ultra-low iridium loading of 0.08 mg cm(-2). Analysis of the IrOx structure and the electrochemical performance revealed three key factors for balancing high stability and activity: (1) high ratio of Ir (IV) to Ir (III) species and high content of hydroxide on the surface; (2) high anodic charge and surface area due to nano-size IrOx particles that are well-dispersed in the Nafion ionomer electrolyte; (3) homogeneous anode catalyst layer morphology.