Considering the importance of rainbow trout ( Oncorhynchus mykiss ) in supplying required protein of people and effort to increase the efficiency of these fish reproduction, some related factors such as sperm quality and potential fertility of male are necessary. The aim of this study was to find out the effects of different dosages of Arginine on the biochemical parameters (including LDH, AST, ALT, ALP, iron, magnesium, phosphorous, chloride, calcium, sodium, Potassium, cholesterol, uric acid, urea, fructose, glucose, total protein and pH) of rainbow trout seminal plasma. For this purpose, five practical diets (each consisting of 3 triplicates) were supplemented with Arginine at 0.00 (Control), 0.50, 1.00, 1.50 and 2.00%. Broodstock feed last for 90 days. At the end of the feeding period one fish was captured from each replicate in order to collect their semen. Results indicate that there were no significant differences in LDH, ALP, Fe 2+ and P content among different treatments. The lowest level of AST and ALT and the highest level of Ca 2+ and Mg 2+ ions were obsereved in the treatment fed with 1.50% of Arginine which showed significant differences with other treatments (p 0.05). Moreover, the amount of Cl - , Na + and K + ions were significantly increased in the seminal plasma in fish which were fed on diets containing arginine in comparison to control. As the amount of Arginine were increased, the levels of uric acid stepped up significantly in contrast to the urea and glucose. The highest amounts of cholesterol, fructose and total protein were observed in the treatments fed on 2.00, 0.50 and 1.00% of Arginine, respectively, that showed significant differences with other treatments (p 0.05). The highest pH value was assayed in the 1.50% of Arginine treatment. The results of the Pearson correlation coefficient showed a significant positive correlation between Ca 2+ and Mg 2+ ions and Na + with K + and Cl - ions (r=0.750, r=0.769 and r= 0.938, p 0.01), respectively. On the other hand, a significant negative correlation between cholesterol with Cl - , Na + , K + , ALT and LDH (r=-0.764, r=-0.724 and r=-0.728, p 0.01) and (r=-0.531 and r=-0.560, p 0.05) and also a significant positive correlation between K + and Cl - ions (r=0.836, p 0.01) were observed. Finally, it can be expressed that the levels of most of the ions were increased and there was a reduction in the levels of enzymes in seminal plasma of fish which were fed with practical diet including 1.5% of Arginine. So it can be recommended that adding this value of Arginine to the diets of rainbow trout broodstock, would improve the sperm quality which results in the enhancement of efficiency in rainbow trout reproduction. Keyword: Oncorhynchus mykiss , Arginine, seminal plasma, Biochemical parameter