ZnO nanostructures with various shapes were synthesized under ambient pressure condition by a wet chemical reaction method. Nanorods of ZnO with hexagonal cross-section and their aggregates with radiate shape were synthesized. Precursor concentration affected considerably the shape evolution of ZnO nanorods. Low precursor concentration was proved to be more preferable to the growth of ZnO nanorods, which is attributed to the intrinsic characteristics of chemical reaction in the synthesis of ZnO from zinc compounds.