Effect of different levels of L-carnitine in diets containing different levels of energy and protein on performance, serum components and carcass composition of 360 day old broiler chicks (Ross strain) was investigated up to 56 days of age. A total of 360 broiler chickens were allocated in 2×4 factorial experiment in a completely randomized design with 3 replicates and 15 chickens per replicate. A Ross recommended starter diet was used to feed chickens from 1 to 7 days of age. Birds were fed starter diet (7 to 14 day), grower diet (14 to 28 day) and finisher diet (28 to 56 day). Two levels of energy and protein (low and high) were examined In all of the diets. The levels were recommended by Ross 308 handbook. Four dietary concentrations of L-carnitine (0, 50, 75 and 100 mg/kg) were used in starter, grower and finisher diets. Based on weekly records, the average daily feed intake, average daily gain, body weight and feed conversion ratio of chickens were recorded. At the end of trial, one male and one female chicken from each replicate group were randomly chosen, then were bled via wing vein for measurement of triglyceride, cholesterol, HDL, VLDL and LDL concentrations, and the broiler were weighed and slaughtered to determine the carcass weight, abdominal fat, breast weight percentages and proportional heart and liver weights. For determining the crude fat content samples were provided from liver and breast muscle. For histological experiment one sample of liver was maintained in formalin with %10 concentration. Experimental results indicated that energy and protein density and L-carnitine density had no significant effect on the feed conversion ratio (p 0.05). Supplementation with L-carnitine in the diets increased daily feed intake during the third and seventh week of experiment (p 0.05). Using L-carnitine in diets had no significant effect on body weight and daily gain. However the broiler fed higher energy and protein diets had significantly greater daily gain at fourth week of age and body weight at the end of fourth, five, sixth and seventh week of age (p 0.05). Supplementation with L-carnitine in the diets decreased crude fat content of the liver, percentage of abdominal fat and increased percentage of breast weight and dressing percent (p 0.05). However broilers fed high energy and protein density diets had significantly reduction of abdominal fat content (p 0.05). The serum triglyceride and VLDL concentrations in the carnitine supplemented group were significantly lower than the control group (p 0.05). However high energy and protein levels resulted significant increase in triglyceride and decrease in cholesterol and HDL. There was no interaction effect between carnitine levels and energy and protein density with respect to all the traits. The results of this study demonstrated that independent of dietary energy and protein levels, supplementi