In order to explore the rule of optical properties of ZnO changing with the concentration of solution, nano-ZnO with hexagonal wurtzitc structure is synthesized by hydrothermal method at low temperature. Then, the morphology, structure and optical properties of the samples arc characterized, respectively. The results indicate that nano-ZnO grows preferentially along the (002) plane. And complex of Zn (OH)₄ ^2- is the growth clement of nano-ZnO. The morphology of the samples is influenced by the concentration, temperature and reaction time of the solution. As the concentration ratio of OH^- and Zn²⁺ increases, the sample changes from the rod-like structure to the flaky form, and the phenomenon of reunion appears. Photolumincsccncc (PL) spectra shows that the sample has fine emission ability in red to green band, and it has not only an obvious visible emission band near the wavelength of 560 nm, but also a ultraviolet emission peak near 374 nm. Finally, the luminescence mechanism of the sample is discussed.