Abstract: Objective To study the pathophysiological mechanism of the morphological change of immature pulmonary vessels in the piglet model of congenital heart defect with decreased pulmonary blood flow established with balloon atrial septostomy and pulmonary artery banding. Methods Twenty piglets at an age of one to two months were divided into three groups with random number table. For the control group (group C,n=6), small incisions were carried out on the right chest to produce a transient reduction in the pulmonary blood; for the lowmedium pulmonary artery stenosis group (group T1, n=7), the balloon dilator was delivered through the surface of the right atrium and septostomy and pulmonary artery banding were performed, and the systolic transpulmonary artery banding pressure (Trans-PABP) was controlled to be 20.30 mm Hg; For the severe pulmonary artery stenosis group (group T2, n=7), the same surgical procedures with group T1 were performed while TransPABP was controlled to be more [CM(159mm]than 3050 mm Hg.At 2 months after surgery respectively,a lung tissue of 1.0 cm×0.8 cm×0.8 cm from the lateral segment of the right middle lobe was taken out to be observed under optic microscope. The morphological change of the distal arterioles was detected. Furthermore, the content of vascular endothelial growth factor (VEGF) and matrix metalloproteinase2( MMP2) were also examined by the method of enzymelinked immunosorbent assay (ELISA). Results The model was successfully established in all the survival piglets of the group T1 and group T2. Two months after operation, the inner diameter of the pulmonary arterioles in group T1 was significantly higher than that in group C (82.89±10.72 μm vs.74.12±9.28 μm;t=－5.892, Plt;0.05), so as group T2 (85.47±5.25 μm vs.74.12±9.28 μm;t=-6.325, Plt;0.05); the number of arterioles per square centimeter (NAPSC) of group T1 was significantly lower than that of the group C (229.70±88.00 entries/cm 2 vs. 431.50±40.60 entries/cm2; t=39.526, Plt;0.05), so as group T2 (210.00±40.30 entries/cm2 vs. 431.50±40.60 entries/cm2; t=67.858, Plt;0.05). Two months after operation, the lung expression of MMP -2 and VEGF in group T1 was significantly lower than that in group C (58.30±19.60 ng/ml vs. 81.20±16.70 ng/ml, t=14.261, Plt;0.05; 17.80±3.00 pg/ml vs. 21.40±3.80 pg/ml, t=8.482, P＜0.05), so does group T2 (42.10±15.20 ng/ml vs. 81.20±16.70 ng/ml, t=27.318, P＜0.05; 12.30±3.20 pg/ml vs. 21.40±3.80 pg/ml, t=15.139, P＜0.05). Conclusion Structural remodeling of pulmonary extracellular matrix is an important feature of the piglet model of congenital heart defect with decreased pulmonary blood flow. The arterioles show significant hypoplasia or degradation. Change in the structural proteins and cytokines during the reduction of blood in the lung is the key to structural remodeling.