Abstract: Objectives  To investigate the effect of different dosages of low molecular weight heparin on acute pulmonary  embolism and inhibition of pulmonary intimal hyperplasia in immature rats. Methods 90 male immature SD rats were randomly  divided into five groups: sham group, pulmonary embolism group, low-low molecular heparin group (L-LMH), medium-low  molecular heparin group (M-LMH) and high-low molecular heparin group (H-LMH). The model of acute pulmonary embolism  was established through jugular vein injection with gel-foam solution. The rates in the L-LMH, M-LMH, H-LMH groups were  treated with low molecular weight heparin by subcutaneous injection after surgery with a dosage of 0.005ml/kg, 0.01ml/kg,  0.02ml/kg, twice a day. Animals in the control group were given saline injection. Arterial blood gas, pulmonary artery pressure  (mPAP), right ventricular pressure (RVP), wall area/tube area, wall thickness/tube diameter, and the expression of PDGF-B  and MCP-1 at gene and protein levels in lung tissue were detected on the 7th (7d), 14th (14d) and 28th (28d) after opration.  Results There were significant differences of PaO2 among 5 groups on 7d, 14d and 28d. PaO2 in group M-LMH (105.1±4.6  mm Hg) were significantly higher than that of embolization group, L-LMH, but not H-LMH group at 28d. mPAP of M-LMH  group was lower than that in the other three intervention groups, but showed no significant difference compared with sham group  (P?>0.05). There were significant differences of RVP on 7d and 14d. PDGF-B, MCP-1 of M-LMH group were significantly  lower compared with the other three intervention groups (P?＜?0.05), but showed no significant difference compared with sham  group (P?>0.05). Wall area/tube area, wall thickness/tube diameter scores of M-LMH group had no significance differences  compared with sham group on 28d (P?>?0.05). Conclusion  Medium dose of low molecular weight heparin could ameliorate the  acute pulmonary embolism and inhibit the proliferation of pulmonary arteries in rats.