Despite the fact that Ta3N5 absorbs a major fraction of the visible spectrum, the rapid decrease of photocurrent encountered in water photoelectrolysis over time remains a serious hurdle for the practical application of Ta3N5 photoelectrodes. Here, by employing a Co3O4 nanoparticle water oxidation catalyst (WOC) as well as an alkaline electrolyte, the photostability of Ta3N5 electrode is significantly improved. Co3O4/Ta3N5 photoanode exhibits the best durability against photocorrosion to date, when compared with Co(OH)x/Ta3N5 and IrO2/Ta3N5 photoanodes. Specifically, about 75% of the initial stable photocurrent remains after 2 h irradiation at 1.2 V vs. RHE (reversible hydrogen electrode). Meanwhile, a photocurrent density of 3.1 mA cm−2 has been achieved on Co3O4/Ta3N5 photoanode at 1.2 V vs. RHE with backside illumination under 1 sun AM 1.5 G simulated sunlight. The reason for the relatively high stability is discussed on the basis of electron microscopic observations and photoelectrochemical measurements, and the surface nitrogen content is monitored by X-ray photoelectron spectroscopic analysis.