A novel ZnO hierarchical micro/nanoarchitecture is fabricated by a facile solvothermal approach in an aqueous solution of ethylenediamine (EDA). This complex architecture is of a core/shell structure, composed of dense nanosheet-built networks that stand on a hexagonal-pyramid-like microcrystal (core part). The ZnO hexagonal micropyramid has external surface that consist of a basal plane (000 (1) over bar) and lateral planes {0 (1) over bar 11}. The nanosheets are a uniform thickness of about 10 nm and have a single-crystal structure with sheet-planar surfaces as {2 (1) over bar(1) over bar0} planes. These nanosheets interlace and overlap each other with an angle of 60 degrees or 120 degrees, and assemble into a discernible net- or grid-like morphology (about 100 nm in grid-size) on the micropyramid, which shows a high specific surface area (185.6 m(2) g(-1)). Such a ZnO micro/nanoarchitecture is new in the family of ZnO nanostructures. Its formation depends on the concentration of the EDA solution as well as on the type of zinc source. A two-step sequential growth model is proposed based on observations from a time-dependent morphology evolution process. Importantly, such structured ZnO has shown a strong structure-induced enhancement of photocatalytic performance and has exhibited a much better photocatalytic property and durability for the photodegradation of methyl orange than that of other nanostructured ZnO, such as the powders of nanoparticles, nanosheets, and nanoneedles. This is mainly attributed to its higher surface-to-volume ratio and stability against aggregation. This work not only gives insight into understanding the hierarchical growth behaviour of complex ZnO micro/nanoarchitectures in a solution-phase synthetic system, but also provides an efficient route to enhance the photocatalytic performance of ZnO, which could also be extended to other catalysts, such as the inherently excellent TiO2, if they are of the same hierarchical micro/nanoarchitecture with an open and porous nanostructured surface layer.