Abstract: Objective To investigate the protective effects of angiotensin-converting enzyme（ACE） inhibitor（cilazapril） on circulating endothelial cells（CEC） in rats with chronic hypoxic pulmonary hypertension. Methods Catheter techniques were used to observe the hemodynamic change,microscopy was performed to observe the changes of CEC,and biochemical methods were employed to determine lipid peroxidization（LPO） and ACE. Results The levels of LPO,CEC,right ventricular pressure,systolic pulmonary arterial pressure,and mean pulmonary arterial pressure were increased in the hypoxia group and decreased in the ACE inhibitor（cilazapril） group compared with the placebo group（P＜0.01）. Conclusion Cilazapril as an ACE inhibitor can reduce CEC by inhibiting LPO.