This study was performed in order to investigate effects of CuO nanoparticle (50 nm) and CuSO4, 5H2O as a disinfectant, on growth indices and gill histopathology of grass carp fingerlings. For this aims, 200 grass carp fingerlings (4.5±0.5g), were divided to four treatments included: copper sulfate (1, 1.5 mg/L), CuO–NPs) 100, 200 mg/L) and control group with triplicate, then were studied during 30 days. They were fed with 0.8 French BioMar carp and sampled on, 5 th,10 th, 15th, 20th, 30 th day after exposure. Growth indices of fishes were analyzed by SPSS software. Also, the gill microscopic studies were done. Specific growth rate, condition index, average weight and survival respectively, in copper sulfate 1.5 mg/L group and then CuO-NPs 200 mg/L had significantly decreased (p