Abstract: AIM To observe the protection of L-carnitine (L-CN) on myocardial metabolism and heart function in hypertensive rats complicated by diabetes. METHODS The method of operation (two kidney one clip) plus streptozotocin(STZ) injection was adopted to set up the model of hypertensive rats complicated by diabetes(HP-DM). Thirty male SD white rats were divided into three groups at random: normal group(n=10), HP-DM model group(n=10) and HP-DM treatment group(n=10). After 10 weeks, left ventricular ejection faction(EF), fractional shortening (FS), left ventricular systolic pressure(LVSP) and left ventricular end-diastolic pressure(LVEDP) were determined by echocardiography. Serum level of glucose(GLU), total cholesterol(TC), triglyeride(TG), low-density lipoprotein cholesterol(LDL-C), high-density lipoprotein cholesterol(HDL-C) and free fatly acids(FFA) were measured. The cardiac index(CI=HW/BW) was computed. Myocardium changes were observed by HE staining and electron microscope respectively. RESULTS After10 weeks’ treatment, higher serum levels of TC, TG, LDL-C, GLU, FFA, CI and LVEDP (P＜0.05), and lower HDL-C, EF, FS and LVSP(P＜0.05) were observed in the HP-DM model group in comparison with those in the control group. In the HP-DM group, HE staining revealed that hyertrophy and disordered arrange of myocardial cells, fracture and dissolution of muscle fibers and nuclear fission. Focal fracture, dissolution and necrosis of muscle fibers and swelling and vacuolization of mitochondria could be observed under electron microscopy. Compared with those in the HP-DM model group, the serum levels of LDL-C, GLU, FFA and LVEDP decreased significantly(P＜0.05) and the EF, FS and LVSP increased significantly(P＜0.05) in the HP-DM treatment group. HE staining revealed visible hyertrophy and disordered arrange of myocardial cells. Fracture and dissolution of muscle fibers and nuclear fission, and cardiac muscle ultrastructure was generally normal under electron microscopy in the L-CN treatment group. CONCLUSION L-CN assuages the cardiocyte energy metabolism disorder, decreases the plasma FFA concentration and thus protects the cadiocytes and cardiac function.