Objective Methods MSCs of Wistar rats were isolated and cultivated by bone marrow adherent culture． The cells between the third and fifth passages were prepared for transplantation． 20 male Wistar rats were divided into four groups by the number of MSCs transplanted intravenously: the 5 × 10⁵group、the 1 × 10⁶ one、the 5 × 10⁶ one and the control one ( n= 5) ． RVSPs were measured at the time of pro-transplantation，5 min，30 min and 24 h post-transplantation． The other 40 rats were randomly divided into four groups: ①the MCT /MSCs 5 × 10⁵ group; ②the MCT /MSCs 1 × 10⁶ one; ③the MCT one; ④ the control one． Rats were injected intraperitoneally with 60 mg·kg 1 MCT，except the control group with physiological saline． Phosphate buffered saline with 5 × 10⁵ MSCs and 1 × 10⁶ MSCs were transplanted into rats through external jugular vein in the first two groups respectively． At the forth week after cell administration，RVSPs were measured and lungs’tissues were stained with Hematoxylin-eosin staining，lichen dyed red staining and smooth muscle Actin immunohistochemistry． Lungs’pathological changes were observed． All data were analyzed statistically by one way analysis of variance using SPSS 11. 0． Results It was safe to transplant MSCs less than 1 × 10⁶ intravenously． At the forth week after MSCs administration，RVSP and ventricular ratio significantly decreased in the MCT /MSCs 1 × 10⁶ group compared to that in MCT group ( 47. 2 ± 10. 5 mmHg vs 35. 8 ± 8. 4 mmHg，0. 3572 ± 0. 0923 vs 0. 4454 ± 0. 0935，respectively，both P＜0. 05) ，but they didn’t differ statistically in MCT /MSCs 5 × 10⁵ group compared to in the MCT one． By pathological staining，medium thickness of the pulmonary arterioles was observed comparatively thinner in the MCT /MSCs 1 × 10⁶ group than those in the MCT group． In contrast they had no significant difference between in the MCT /MSCs 5 × 10⁵ group and in the MCT one ( P＞0. 05 ) ． Conclusions Intravenous MSCs administration can prevent lungs injury induced by monocrotaline and 1 × 10⁶ MSCs administration has better effects than 5 × 10⁵ MSCs’．