Photocleavage of H2O into clean and storable H-2 fuel by photoelectrochemical (PEC) cell is a vital part of the sustainable hydrogen economy. However, thus far one of the limitations confronted by PEC cell to preferable performance is the insufficient behavior of photoanode for water oxidation half-reaction. One of the strategies to elevate the photoanode performance is integrating with an oxygen evolution catalyst (OEC) to remove the bottleneck of the water oxidation kinetics. Herein, an ultrafine cobalt iron oxide (CIO) nanocrystalline is reported as a novel OEC for photoelectrochemical water splitting. The CIO evenly distributing on the surface of hematite nanorod arrays not only greatly facilitates the surface hole injection, but also promotes the charge separation along with passivating the surface states. Such combined effects of CIO finally lead to an impressive 1.71 fold enhancement on the photocurrent density at 1.23 V-RHE and approximate to 170 mV negative shift of onset potential, even overwhelms the commonly utilized Co-Pi. Along with its excellent long-term stability, the CIO possesses a great potential application in PEC water splitting devices.