120 specimens of common carp were treated in salt water with 4ppt, 8ppt, 12ppt treatments and freshwater with three replications for 2 weeks. For histomorphology study in the end of the period  thickness of 4μ to 6μ were prepared. Also samples were collected for immunohistochemical and ultrastructural studies. Results showed that at the filament position, the highest and lowest number of chloride cells belonged to 8 ppt salinity and control treatment, respectively. Also at this position, the largest size of the cells belonged to the salinity of 12 ppt and the smallest size belonging to 4 ppt. At the lamellar site, the highest number of chloride cells belonged to 4 ppt salinity and the lowest number belonged to the control treatment without changing in the size. In immunohistochemical studies for Na⁺/K⁺-ATPase localization, it was found that chloride cells were high in filament epithelium and few in the lamellar epithelium in different salinities. These cells were present with intense immunofluorescence in the apex, base, and between inter lamellar filament. According to the electron microscopy studies, the membrane of the chloride cells of the gill of common carp showed that different apical morphologies as the deep hole, shallow basin and wavy convex. In general, range of tolerance and adaptation of common carp is high to salinity changes due to rapid changes in chloride cells in the direction of entering and leaving water and electrolytes. So that, this species was able to adapt to the changes in environmental different salinity and to survive.