Abstract: Objective To observe the effect of L-carnitine on TGF-β/Smads signal pathway and myocardial energy metabolism in rats with myocardial hypertrophy anddiscuss its related mechanism. Methods Forty healthy SD rats were randomlydivided into blank control group, model group, L-carnitine lowdose group (50 mg/kg) and highdose group (100 mg/kg). Myocardial hypertrophy model was established in model group and L-carnitine group with continuous injection of isoproterenol (Iso) for 5 mg/(kg·d) for 10days. L-carnitine was injected in rats of L-carnitine group since the secondday after modeling for eight weeks. Then the heart mass index (HMI) and left ventricular mass index (LVMI) were measured, myocardial cells were observed under light microscope by HE staining. The free fatty acids (FFA), lactate acids (LAC) and adenosine triphosphate (ATP) contents in myocardial tissue of rats weredetected, and the expression of TGF-β1 and Smad3 protein weredetected by Western Blot. Results The HMI and LVMI of model group, L-carnitine lowdose group and highdose group increased significantly when compared to blank control group (P< 0.05), myocardial cells were more hypertrophied and its structure became moredisordered compared to blank control group. The content of FFA and LAC were higher than blank control group while ATPdecreased (P< 0.05), and the expression of TGF-β1 and Smad3 protein increased significantly (P< 0.05). The content of ATP increased significantly while all other indexesdecreased significantly in L-carnitine lowdose group and highdose group when compared with model group (P< 0.05). Conclusion L-carnitine improves the energy metabolism of myocardial hypertrophy and restrains TGF-β/Smad3 signal pathway, thus it plays a protective role in the myocardial cells by reducing the expression of TGF-β1 and Smad3 protein.