In order to explore the toxicity of nano ZnO and its relationship with the release of Zn²⁺ and its own characteristics, zebrafish were exposed to the nano-ZnO, bulk-ZnO, and ZnSO₄ suspension for 4, 24 and 96 hours by using the acute toxicity test, and the glutathione-s-transferase (GST), catalase (CAT), and the levels of glutathione (GSH) and reactive oxygen species (ROS), the relative expression levels of Bax, Bcl-2, TNF- alpha and IL-6 in liver, intestine and gill of zebrafish were measured. The objective of this study was to compare the the strength of oxidative stress effects of nano-ZnO, bulk-ZnO and ZnSO₄ in zebrafish, and discuss whether the toxicity of nano-ZnO is related to the releasing Zn²⁺ or nanoparticles itself. Results showed that nano-ZnO, bulk-ZnO, and ZnSO₄ can cause oxidative stress in the tissues of zebrafish, and the nano-ZnO induced more oxidative damage than the other two forms of zinc. Consequently, we can conclude that the toxicity of nano-ZnO was higher than that of bulk-ZnO and ZnSO₄ on oxidative stress in zebrafish and the toxicity of nano-ZnO was mainly attributed to the nanoparticles itself.