ZnO nanocrystals were synthesized using the continuous hot compressed water technique at different temperatures and reaction times. The size, crystallinity and morphology of the ZnO nanocrystals were characterized using scanning electron microscopy, transmission electron microscopy, and X-ray diffraction techniques. Star-like, spherical nanoparticles with diameters varying from 10 to 200 nm and rod-like structures with lengths up to 300 nm were obtained. Tubular ZnO nanocrystals were obtained when oleic acid was added in situ into the feeding solutions during the reaction. The corresponding growth mechanisms were elucidated. A scaled-up process (output of ZnO: 1 kg/h) produced ZnO nanocrystals of a similar size to those produced on the small scale. (C) 2013 Elsevier B.V. All rights reserved.