Zinc oxide nanoparticles (ZnO-NPs) have widespread applications in various industries and cosmetics that raise concerns about their hazards to the environment. Eco-toxicological data show that nanoscale zinc oxide has harmful impacts on both aquatic species and human health. The co-precipitation method was followed to synthesize ZnO-NPs and X-ray diffraction (XRD) technique and scanning electron microscope (SEM) were used for characterization. The acute toxicity at 96h was determined by using Bighead carp as a model fish after the exposure to 11 various concentrations (2-50mg/L) of ZnO-NPs. The mean 96-h LC₅₀ was measured as 22.24mg/L. Bighead carp showed different behavioral changes that lead to death during acute toxicity tests. Oxidative stress in terms of lipid peroxidation (LPO) and superoxide dismutase (SOD) was determined in fish gills and liver after chronic exposure to ZnO-NPs for 90 days and sampling was done at 15, 30, 45, 60, 75 and 90-day intervals. Significant alterations in SOD and LPO levels were determined in tissues of the liver and gills as compared to the control group. We conclude that induced toxicity mechanism of ZnO-NPs suspension in freshwater environments may be due to elevated oxidative stress.