Widespread attention has recently been paid to artificial photosynthesis as a promising energy-providing system for the future to produce green and storable hydrogen fuel. The efficiency of hydrogen generation from photoelectrochemical and electrochemical water splitting is severely limited by the sluggish kinetics of oxidation of water. Therefore, the development of efficient electrocatalysts for water oxidation is an avenue to improve the competence of a fuel generation technology. Iridium oxide (IrOx) catalysts have been widely studied for water oxidation in both electrochemical and photochemical systems, owing to its actuation over a broad pH range, low resistivity, and superior chemical/thermal stability. Nanostructure control of IrOx is important to develop high activity water oxidation catalyst based on IrOx. We reported the formation of a mesoporous IrOx thin film using an amphiphilic block polymer as a template, which has a high specific surface area (105 m2g-1). IrOx powder is widely used, but IrOx powder having a specific surface area of 100 m2g-1 or more has not been developed so far. Herein we report to create IrOx powders with a high specific surface area without using templates.