Unusual designated-tailoring on the preferential growth of ZnO meso-/nanocrystals with different shapes and sizes was successfully achieved through a very facile precursor-hydrolysis process in a water/organic solvent system. The shape-evolution of ZnO from mesocrystal to nanocrystal was achieved by increasing the amount of water in the reaction system under otherwise the same conditions, which can be attributed to selective-capping and preferential growth mechanisms based on DFT calculations. Typically, nanoparticle-aggregated cone-like ZnO mesocrystals enclosed by curved (101) and flat (000) surfaces were prepared in a lower amount of water, while various ZnO crystals without curved (101) surfaces were synthesized in a higher amount of water. In particular, “surface heterojunctions” were constructed in cone-like ZnO mesocrystals with co-exposed curved (101) and flat (000) surfaces, and these displayed greater photocatalytic degradation of methyl orange dye compared to the ZnO nanocrystals. This investigation is of great significance to the synthesis of additive-free ZnO mesocrystals, and offers a good opportunity to understand the formation mechanism and fundamental significance of the preferential growth of ZnO mesocrystals. Importantly, it is believed that the water-induced synthetic strategy reported here could provide a promising way to design more and more metal oxide architectures with controllable morphologies.