NaTaO3 loaded with NiO cocatalyst is one of the few photocatalysts for overall water splitting in UV region, which have attracted much attention. In this work, density functional theory calculations have been performed to investigate the interfacial geometries, electronic structures, charge transport, optical absorptions and band offsets of NiO(001)/NaTaO3(001) slab models. By considering possible terminations of NaTaO3(001) surface, two heterostructures denoted as NiO/TaO2 and NiO/NaO have been constructed. Our results show that two kinds of contact are thermodynamically stable, and there is a stronger rumpling of atomic layers appearing in NiO/NaO than NiO/TaO2. The calculated band structures reveal that NiO(001)/NaTaO3(001) interfaces have indirect band gaps. The mobilities of photo-induced charge carriers in interfacial structures are faster than those in pure surfaces. NiO/TaO2 has a higher mobility and lower recombination rate of photo- generated electrons and holes than NiO/NaO. Loading NiO on NaTaO3 surface has a negligible effect on the extension of light absorption, which is consistent with experiments. Both heterostructures form a Type-II band alignment. The difference of electrostatic potentials around the interface as a driving force boosts the migration of electrons and holes to different domains of the interface, which is beneficial to extend the lifetime of photo induced carriers and improve the photocatalytic activity of NaTaO3 system. NiO/TaO2 has the ability of overall splitting water with NiO as the oxidation cocatalyst, while in NiO/NaO, the photo-generated electrons and holes are accumulated on NiO and NaO side, respectively. Our results demonstrate that the function of NiO in NiO/NaTaO3 photocatalytic system is determined by the termination property of NaTaO3 (001) surface, which may be one possible reason why it is difficult to ascertain whether NiO is a proton reduction cocatalyst or water oxidation cocatalyst experimentally. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.