The preventive effect of Radix Salciae Miltiorrhizae (RSM) on pulmonary hypertension (PHT) and right ventricular hypertrophy (RVH) induced by monocrotaline (MCT) in rats was observed with the methods of measuring the right ventricular systolic pressure (PVSP), the ratio of the right ventricle to the left ventricle plus interventricular septum RV/(LV+S) and the pulmonary small artery morphological analysis. The results show that RSM can reduce the PHT, and prevent the RVH and the increase of the medial thickness of pulmonary small arteries. It can also prevent the endothelial cell injury produced by MCT. The pharmacological effects of RSM on pulmonary circulation were also discussed.