Inroduction & Objective: The changes in environmental factors such as salinity are considered as a stressful factor and a negative effect on the growth and survival of aquatic animals. Gill chloride cells play a major role in the exchange of ions in fish. Therefore, the purpose of this study was to evaluate the changes of number and area chloride cells in response to increase salinity. Materials and Methods: 180 rainbow trout with an average initial weight of 28. 95 ± 1. 91 grams were transferred to 15, 20 and 25 ppt salt waters for 60 days. Adjustment period to saline water was carried out at 15 days. After exposure to different salinity for 7, 15, 30, 45 and 60 days, three fish’ s gill from each experimental unit dissected for evaluating the number and area of chloride cells. 5-7 µ thick cross-sections of tissue is obtained using haematoxylin and eosin. Results: After 7 days, the highest number and area of chloride cells were observed in 25 ppt salinity treatment. In addition, the number and size of chloride cells were significantly higher in salinity treatments compared to control groups from 15 to 60 days. Fish in 25 ppt treatment were gradually killed until the end of the exposure period. Conclusion: In this study, chloride cells of rainbow trout were more observed in the base of filaments and played an important role in ion secretion