In the present study, acute toxicity of ZnO NPs and silica-coated ZnO NPs following intravenous injection using mice were evaluated. ZnO dispersion （average size: 148 nm） at a dose of 10 mg/kg and 30 mg/kg, and a Silica-coated ZnO （ZnOSiO2）at a dose of 10 mg/kg, 30 mg/kg and 130 mg/kg were injected intravenously via tail vein of mouse. All mice （n = 6） died within 5 min following intravenous ZnO dose of 10 mg/kg and 30 mg/kg. Impaired motility and convulsion were observed before the death. On the other hand, silica-coating can reduce the toxicity. All mice survived and impaired motility was not observed following intravenous ZnO-SiO2 （average size: 81 nm） treatment at the same dose of ZnO NPs. At a dose of 130 mg/kg, impaired motility and convulsion was observedin all mice, one mouse out of 5 mice died after 5 min. Blood coagulation was observed in all died mice,which may be related to the toxicity of ZnO NPs.