This paper focuses on the isolation of 3,5,7,4’-tetrahydroxyflavone (Kaempferol)from the methanol extract of Cassia Alata leaves and on the study of its pharmacological effect in vitro on pulmonary artery smooth muscle cells (PASMC). This flavone was purified by column chromatography on silica gel and eluted with hexane/ethyl acetate in proportions of increasing polarity. Its chemical structure was elucidated by 1H-NMR and 13C-NMR spectroscopies. Pharmacological tests of 3,5,7,4’-tretrahydroxyflavone were carried out on PA-SMC, which were isolated from patients suffering from Pulmonary Arterial Hypertension (PAH), by using the BrdU (5-bromo-2-deoxyuridine)in corporation method. This allows to investigate the ability of 3,5,7,4’-tetrahydroxyflavone to inhibit cell proliferation.The obtained results showed an anti-proliferative effect of kaempferol at a concentration of10g/mL. In addition, it should be noted that, in the same condition, a similar result was observed with the 2-hydroxy-Narylbenzamidederivativethat we have previously synthesized. This suggest that the kaempferol showed a significant marked inhibition of pulmonary artery smooth muscle cells proliferation (P<0.0001). It appears to have an inhibitory effect closely similar to that of 2-hydroxy-N-arylbenzamide derivative. Therefore, 3,5,7,4’- tetrahydroxyflavone (kaempferol) could be a potential alternative treatment for Pulmonary Arterial Hypertension diseases.