Background: Tadalafil, phosphodiesterase 5 (PDE5) inhibitor, has been reported to have a therapeutic effect on pulmonary hypertension (PH), and we reported the preventive effect of tadalafil on monocrotaline (MCT) induced PH in rats. Purpose and Methods: The aim of this study is to evaluate the effect of tadalafil on pulmonary artery and compare with echocardiographic findings. We used 4-week-old male SD rats which developed PH from about 7-week-old by 5-time subcutaneous injection of MCT. We evaluated heart function by echocardiography before and after administration of tadalafil (9.5 mg/kg/day) or saline (vehicle group) for 6 times every other day. After verification of survival rate at 8-week-old, we measured the weight of RV, LV and the lung. We evaluated the thickening of pulmonary artery to calculate the ratio of external to internal diameter in hematoxylin-eosin stained lung tissues. Results: The survival rate of tadalafil treatment group at 8-week-old was higher than that of the vehicle group. Just before tadalafil treatment, there was no evidence of PH. At 8-week-old, the ratio of RVEDA (EDA: end-diastolic area) to LVEDA, the peak pressure gradient across RV-RA, AT/ET (the ratio of acceleration to ejection time) and the ratio of RV to LV weight corrected by body weight were significantly smaller in the tadalafil treatment group than those in the vehicle group (Table). The ratio of external to internal diameter of pulmonary artery in the vehicle group was significantly smaller than that in the tadalafil treatment group and well correlated with the ratio of RVEDA to LVEDA (Figure). Conclusion: Tadalafil could have a therapeutic effect on right ventricular failure to prevent the thickening of pulmonary artery wall.