High-quality and controllable growth of nanocrystals (NCs) have been attracting great attention. Here, a high-temperature-mixing hydrothermal (HTMH) method was designed to synthesize ZnO NCs with high crystallinity and narrow size distribution in a wide growth window. Compared with conventional hydrothermal (CH) growth, zinc source and alkali precursors were intentionally separated in temperature-rising stage and permitted to mix at the starting of heat preservation stage of HTMH growth. Highly crystalline ZnO NCs with uniform spherical morphology can be formed at alkali concentration and temperature windows as wide as 0.1-0.5 M and 160-200 degrees C, respectively. However, the products via CH method have much larger changes in not only morphology but also size. These results demonstrated that the high-temperature-mixing reaction greatly facilitates nucleation but depresses grain growth. Considering the simplicity and reproducibility, such HTMH method could have wide potentials for the fabrication of various functional nanocrystals. (C) 2013 Elsevier B.V. All rights reserved.