The present study was conducted to examine the effects of L-carnitine on pulmonary hypertensive response in broiler chickens reared at high altitude and exposed to hypobaric hypoxia. A total of 192 day-old male broilers (Cobb 500) were randomly assigned to 4 treatments and 4 replicates of 12 chicks. A basal diet composed of mainly corn and soybean meal was formulated and served as a control. Three additional treatments were made by supplementing graded levels of L-carnitine (50, 100, and 150 mg/kg). Chicks received dietary treatments at free access from 1 to 42 days of age. Results indicated that feeding L-carnitine at 100 mg/kg caused a significant increase in plasma nitric oxide (NO) with concomitant decrease in plasma malonedialdehyde (MDA). The Lead II electrocardiogram indicated reductions of S wave amplitude for all three doses of L-carnitine relative to the control and the difference between the birds that received L-carnitine at 50 mg/kg and the control was significant (P＜0.05). The right ventricular weight ratio (RV/TV) tended to decline when L-carnitine supplemented to diets. In conclusion, L-carnitine reduced ascites mortality in broiler chickens by increased NO production, reduced MDA concentration, and reduced right ventricular hypertrophy.