In order to determine of some blood serum biochemical parameters in rainbow trout, blood samples were taken from the caudal vein of 531 apparently healthy rainbow trout cultured in earthen ponds. Data was analyzes by regression analysis, pearson correlation coefficient test and fisher's test. The mean values of ALP, AST, ALT and LDH were 722±320, 337±150, 29±24 and 579±353 U/L and the mean values of sodium, potassium and chloride were 144.4±6.8, 1.8±0.5 and 126.7±13.0 mmol/l respectively. The mean values of total protein and albumin were 4.0± 0.7 and 1.7±0.8 gr/dl. In this study the mean values of uric acid, creatinine, urea, triglyceride, cholesterol, glucose, calcium and phosphorus were 1.7±0.6, 0.4±0.2, 9.1±4.8, 273.4±165.2, 270±88, 103.8±35.9, 15.9± 2.9 and 25.3±5.0 mg/dl respectively. The results of regrisson analysis showed that with increase in age, the mean values of albumin, total protein, cholesterol, sodium, potassium and chloride increased and the mean values of triglyceride, creatinin, AST, ALT and LDH decreased (p<0.05).