A novel cellulose-derived hierarchical g-C3N4/TiO2-nanotube heterostructured nanocomposite was fabricated by in situ coating thin g-C3N4 layers onto the surfaces of the TiO2 nanotubes, which were synthesized by utilizing the natural cellulose substance (e.g., commercial ordinary filter paper) as the structural template. These g-C3N4/TiO2-nanotube composites with varied thicknesses (ca. 3-30 nm) of the outer g-C3N4 layers displayed improved visible-light (lambda > 420 nm)-driven photocatalytic degradation performances toward methylene blue. The optimal nanocomposite with an outer g-C(3)N(4)4 layer of ca. 7.5 nm composed of 46 wt % g-C3N4 displayed an apparent rate constant of 0.0035 min(-1), which was 8.5- and 4-fold larger than those of the referential TiO2-nanotube and g-C3N4 powder. The excellent and durable photocatalytic activities of these cellulose-derived g-C3N4/TiO2-nanotube composites were ascribed to their hierarchically network porous structures replicated from the cellulose template, as well as the formation of close heterojunctions in-between the g-C3N4 and TiO2 phases. Moreover, it was demonstrated that the photocatalytic mechanism matched with the type-II heterostructured model, while the main effective species during the photocatalytic processes of the nanocomposite were proved to be superoxide radicals.