In the present study medicinal plants of Cassia fistula L were collected from Thanjavur, Thanjavur (Dt), Tamilnadu. In vitro antibacterial activity were performed by agar well diffusion method against five human pathogenic bacterial cultures such as Bacillus subtilis, Escherichia coli, Klebsiella pneumoniae, Proteus vulgaris and Staphylococcus aureus. The methanolic leaves extract of Cassia fistula L. showed highest antibacterial activity against Escherichia coli. Phytochemical analysis of leaves extracts of Cassia fistula L. revealed the presence of alkaloids, phenols, proteins, saponins, sterols and tannins. Antibiotics are an essential part of medicine. However, bacterial resistance emerges and reduce efficacy of antibiotics in the human population.