Systemic symmetry is introduced into our work to investigate the symmetries of different ZnO nanocrystals. Result shows that systemic symmetries obey the law of spontaneous symmetry breaking during the formation of ZnO nanocrystals. A unitary formation mechanism of ZnO nanocrystals is proposed accordingly. Our results provide new insight into both crystallization and spontaneous symmetry breaking at meso-scale. (C) 2011 Elsevier B.V. All rights reserved.