Objective To investigate the effect of bethaneline on expression of rhoA/ROCK pathway protein in pulmonary arterial hypertension （PAH） induced by monocrotaline （MCT） in rats. Methods 72 SD （Sprague Dawley） rats were randomly divided into control group， MCT（50 mg/kg） group， sildenafil （30 mg/kg） group and low， medium， high （100，200，400 mg/kg） group depending on betaine level. Monocrotaline group was adminstered on the 22nd day after successful modeling， and then the right ventricular systolic pressure， right ventricular hypertrophy index and visceral index were measured on the 41st day. The expressions of RhoA， Rock1 and Rock2 in the rat myocardial and lung homogenates were detected by Western blot. Results Compared with MCT， both betaine （400 mg/kg） and sildenafil （30 mg/kg） significantly reduced the mean pulmonary artery pressure （mPAP）， right ventricular systolic pressure （RVSP） and right ventricular hypertrophy index （RVHI）（P < 0.01）. Lung and cardiac tissue expression of RhoA， ROCK1， and ROCK2 proteins were upregulated in the MCT group as compared with the control group， but the expression was significantly suppressed in the betaine group （400 mg/kg）（P < 0.05）. Compared with the control group， the expression of RhoA， ROCK1 and ROCK2 proteins in the lung and heart tissues of the MCT group was up-regulated， but the expression was significantly inhibited in the betaine group （400 mg/kg）（P < 0.05）. Conclusion Betaine can reduce monocrotaline induced pulmonary hypertension in rats. It is speculated that its mechanism may be related to the down-regulated expression of RhoA， ROCK1 and ROCK2 in rat lung and myocardium through the RhoA/ROCK pathway.