Technological and industrial development in recent decades led to large inputs of pollutants to the environment. Due to the harmful effect of pollutants on the environment and living organisms, investigate and prove the damage caused by them is necessary. The aim of this research was to study the pobable toxicity of Cuo nanoparticles (NPs) in the juvenile Cyprinus carpio by evaluating of hematological parameters and histopathological effects. Juvenile carps were treated in 12 aquariums containing the sublethal concentrations of 50, 100 and 200 mg.L-1 of CuO NPs and a control group (without CuO NPs) in three replicates. Fish were exposed to CuO NPs for 14 days. Blood samples were collected after 7 and 14 days and hemoglobin (Hb), hematocrit (Hct), red blood cells (RBC), white blood cells (WBC), lymphocytes, monocytes, eosinophil and neutrophils were measured. Also, MCV, MCH and MCHC were calculated. Tissue sections were prepared from fish gills. Tissue damage was evaluated. The analysis of hematological indices showed that CuO NPs significantly reduced the RBC, Hb, Hct (in 100 and 200 ppm), MCH (in 100 ppm) and MCV (in 50 and 200 ppm) after 14 days (P< 0.05). CuO NPs significantly affected the counts of lymphocytes, monocytes and neutrophils (P< 0.05). The results of gill histopathology showed alterations such as hyperemia, hemorrhage, hyperplasia and adhesion with high intensity and destroyed primary filaments, cell necrosis and telangiectasia with less intensy. Thus it can be concluded that copper oxide nanoparticles at concentrations examined in this study had negative effects on Juvenile carp.