In this study, the effects of CaCl2 (2%) supplemented diet, rock salt (RS, 2%) supplemented diet and control diet (no supplement) on the weight gain, specific growth rate, daily feed intake, feed conversion rate, protein efficiency rate and serum mineral concentration (Na, K, Cl) were investigated in rainbow trout. RS was used for increasing the CAD and Na/K ratio of the control diet and CaCl2 was used for decreasing the CAD level of the control diet. A total of 300 fish, 2 replications for each treatment consisting of 50 fish with a mean initial body weight of 88.2 g were used in the experiment. The experiment lasted fifteen weeks. The diet supplemented with 2% RS improved significantly the weight gain, feed conversion rate and protein efficiency rate in rainbow trout compared to 2% CaCl2 supplemented group and control (P<0.05). Dietary RS supplementation also improved the specific growth rate and daily feed intake. However, this improvement was not statistically significant (P>0.05). Dietary 2% CaCl2 supplementation did not significantly affect the weight gain, specific growth rate, daily feed intake, feed conversion rate and protein efficiency rate in rainbow trout compared to control diet (P>0.05). Na, K and Cl level of the serum and mortality rate were not statistically different among the groups (P>0.05). The dietary 2% RS supplementation produced a better growth and feed efficiency for rainbow trout compared to dietary 2% CaCl2 supplementation and control diet. Added RS to diet was appropriate for making cationic the diet and increasing the CAD level and Na/K ratio of diet.