The study was conducted to determine the effects of dietary L-carnitine and coenzyme Q10 (CoQ10) supplementation on growth performance and ascites mortality of broilers. A 3 x 3 factorial arrangement was employed with three levels (0, 75 and 150 mg/kg) of L-carnitine and three levels of CoQ10 (0, 20 and 40 mg/kg) supplementation during the experiment. Five hundred and forty one-day-old Arbor Acre male broiler chicks were randomly allocated into nine groups with six replicates each. All birds were fed with the basal diets from day 1 to 7 and changed to the experimental diets from day 8. During day 15 to 21 all the birds were exposed to low ambient temperature (15-18 degrees C) to induce ascites. The results showed that under this condition, growth performance of broilers were not significantly affected by CoQ10 or L-carnitine + CoQ10 supplementation during week 0-3 and 0-6, but body weight gain (BWG) of broilers was significantly reduced by 150 mg/ kg L-carnitine during week 0-6. Packed cell volume (PCV) of broilers was significantly decreased by L-carnitine and L-carnitine + CoQ10 supplementation (P < 0.05). Erythrocyte osmotic fragility (EOF), ascites heart index (AHI) and ascites mortality of broilers were significantly decreased by L-carnitine, CoQ10 and L-carnitine + CoQ10 supplementation. Though no significant changes were observed in total antioxidative capability (T-AOC), total superoxide dismutase (T-SOD) was increased by L-carnitine, CoQ10 and L-carnitine + CoQ10 supplementation (P < 0.05). Malonaldehyde (MDA) content was significantly decreased by CoQ10 and L-carnitine + CoQ10 supplementation. The results indicate that dietary L-carnitine and CoQ10 supplementation reduce ascites mortality of broilers; the reason may be partially associated with their antioxidative effects.