Abstract: The study was aimed to investigate the antibacterial activity of Cassia siamea leaves extracts against some bacterial isolates from infantile diarrhoeal samples. The ethanolic extracts of Senna siamea leaves were prepared, screened for phytochemical analysis and tested for its antibacterial activity against 3 pathogenic bacteria (Shigella spp, Escherichia coli, and staphylococcus aurous) recovered from infant diarrhoeal samples of patients attending General Sani Abacha Specialist Hospital Damaturu. Phytochemical constituents found in Cassia siamea leaves extract include; alkaloids, saponins, tannins, steroids and flavonoids as well as the Anthraquinones and phenols were absent. Based on the susceptibility of the organisms to the extracts, staphylococcus aureous was found to be the highest susceptible organisms with average zone of inhibition of 15. 00 (40gm/ml) followed Escherichia coli 15.00 (50gm/ml), and Shigella14.00 (30gm/ml). The MIC of the extracts ranged from 4.125 to 50mg/ml. There was no significant difference on the susceptibility of the organisms against the extracts. The results of the present study have supported the therapeutic potential of Senna siamea and its use as medicinal plant.