BACKGROUND: Heat stress declines the performance through physiologic changes of the poultry. OBJECTIVES: This experiment was conducted to evaluate the effects of different levels of L-carnitine on performance, carcass characteristics and some blood parameters of broiler chickens fed with high fat diets under heat stress. METHODS: Two hundred one-day-old broiler chicks (Ross 308) were used in a completely randomized design by four treatments and five replicates (pen) 10 birds in each replicate. The chickens of experimental treatments were fed with basal diet (control group) and the diets plus the different levels of 100, 200 and 300 mg/kg L-carnitine. The experimental diets were used during the finisher period (day 24 to 42 of age) and under heat stress (32±1ºC as cyclic from 9.00 AM to 5.00 PM). RESULTS: The results showed that the chicks fed with the highest L-carnitine had the higher weight gain during the finisher period as compared to control and the lower L-carnitine fed chicks (p<0.05). Furthermore, the feed conversion ratio of birds fed with the medium and high L-carnitine levels (100 and 300 mg) was lower than that of control birds (p<0.05). In orthogonal contrasts, L-carnitine supplementation caused the improvement in weight gain and feed conversion ratio during both the finisher and whole experimental periods (p<0.05). The consumption of highest L-carnitine decreased the abdominal fat as compared to control (p<0.05). In orthogonal contrasts, L-carnitine consumption caused the higher glucose and lower abdominal fat in comparison to control (p<0.05). CONLUSIONS: Totally, the results of recent experiment show that consumption of 300 mg/kg L-carnitine improves the performance of broiler chickens through the increase in dietary fatty acids and energy.