The toxic impact of copper sulfate in lethal and sublethal concentrations was investigated on gill of grass carp, Ctenopharynogodon IDELLA. In this investigation, 27 specimens were used as control group and three treatment groups were treated with 2. 5 and 5mg/l of copper sulfate. Gill samples were collected from each treatment after 96hr and lesions were analyzed by light microscopy. In histopathological study of the gill tissues, hyperplasia was clearly obvious in treatment specimens. In all of the treated groups, heavy gill-mucus response was observed, which indicate a direct relation with high concentrations. Also, in histological study of the gill, epithelial cells faced to hyperplasia, which increased with high copper densities. Primary lamellar cells wrinkling and also changing in formation were observed in chloride cells. This lesion enhanced in higher densities and in concentrations of 2. 5 and 5mg/l, primary and secondary lamellar epithelial cells were degenerated.