Experiments found that cocatalysts play an important role in influencing the efficiency of photocatalysis and photoelectrochemical water splitting. However, how to choose proper cocatalysts in certain photo catalytic system is still a challenging question. In this work, the first-principles density functional theory is employed to explore two photocatalysts with different photocatalytic activity, CoOx and NiOx loaded Ta3N5, including finding the stable structure of metal oxide adsorbed surface, analyzing the electronic properties, and investigating the behavior of water adsorption. Our results indicate that the structural match between cluster and surface and less distortion of interfacial structure are benefit to the stability of the whole system. Water dissociation tends to occur at the interface between metal oxide cluster and Ta3N5 surface. Combining observations in experiments with our calculated.results, we propose that the obvious difference of photocatalytic activities in CoOx/Ta3N5 and NiOx/Ta3N5 is possibly related to whether there are impurity states located in the middle of band gap, which has adverse effect on the separation of photo-generated electrons and holes. (C) 2017 Elsevier B.V. All rights reserved.