Semiconductor building blocks of varying dimensionality have been used to prepare multifunctional heterostructural photocatalysts that can efficiently separate photo-generated carriers to enhance photocatalytic activity, while exposing the desired reactive facets of the semiconductor via structure engineering remains a fascinating strategy. In this work, ultrathin graphitic-carbon nitride (g-C3N4) nanosheets (NSs) and NiO nanorods (NRs) with exposed {110} facets were utilized as assembly units to strategically construct well-defined g-C3N4@NiO heterostructure via a facile self-assembly, and g-C3N4 NSs were uniformly and intimately enwrapped on the NiO NRs. This shell nanostructure possessed remarkably high photocatalytic activity in the oxidation of organic compounds and in photoelectrochemical reactions. The underlying reasons accounting for the considerably improved photocatalytic oxidation performances of g-C3N4@NiO heterostructure were examined, which resulted from the more efficient charge separation/transfer thanks to the peculiar structural merits of g-C3N4 NSs and NiO NRs, as well as from the intimate interfacial contact and unique spatial integration mode between them. The predominant active species in the photocatalytic oxidation were determined unambiguously, which enabled to elucidate the mechanism of the photocatalytic oxidation.