In this study, the effect of dietary L-carnitine on growth, proximate and muscle fatty acid compositions of rainbow trout (Oncorhynchus mykiss) were investigated. The fish were fed with diets containing 300 mg kg(-1) L-carnitine (LC300), other group was fed with diets containing 600 mg kg(-1) L-carnitine (LC600) and control group was not supplementary L-carnitine for 63 days. The weight gain of LC600 fed with L-carnitine supplemented was found to be 7.73% higher than in control group. Feed conversion ratio in LC600 (1.66) and LC300 (1.60) is better than in control group (2.00). L-carnitine supplemented groups would be lower cost of production than control. Economic conversion ratio (ECR) of LC300 (2.71 US.$ kg) would represent a saving of 0.44 US.$ kg(-1) compared to the control. L-carnitine administration did not affect proximate composition of muscle except for protein percent. Protein content of L-carnitine supplemented groups was higher than control group. Total saturated monounsaturated and polyunsaturated muscle fatty acid content increased by L-carnitine supplementation in rainbow trout. The results indicate that supplementation of 600 mg L-carnitine/kg diet is more effective on growth than the other L-carnitine supplementation level 300 mg kg(-1) and control diet in commercial cage conditions.