The emergence of antibiotic resistance has forced scientists to use natural products to fight against microbes. Cassia fistula L. has been used in traditional medicine to treat various diseases such as diarrhoea, skin diseases, fever, abdominal pain, leprosy, etc. The present study focussed on the growth inhibitory activity of C. fistula leaf ethanol extract against Escherichia coli DH5αstrain. The antibacterial activity of three different concentrations (2, 1, and 0.5%) of C. fistula leaf extract was investigated. The results showed that the extract inhibited the growth of E. colisignificantly. The density of E. coli in the control group was 1.97×109 cells/mL. The number of cells in the treatment at 2% extract concentration was significantly reduced; after 24 hours, 1.04×108 cells/mL were detected. In the treatments at 1% and 0.5% concentrations, there were noticeably fewer cells than in the control (6.48 x 108 and 7.04 × 108 cells/mL, respectively). It was found that the extracts' antibacterial effectiveness varied in a dose-dependent manner. Several secondary metabolites were present in C. fistula, which contributed to its antibacterial action.