The antibacterial effect of ethanolic leaf extract of Costus afer was evaluated on bacterial strains which included Streptococcus pneumoniae, Streptococcus pyogenes and Staphylococcus aureus, among the Gram-positive organisms and Klebsiella pneumoniae, Pseudomonas aeruginosa, Escherichia coli among the Gram-negative, using Standard Microbiological Method [STM]. The in vitro antibacterial activity was performed by Agar-disc Diffusion Method [ADM]. The concentrations used were 150mg/ml, 100mg/ml and 50mg/ml. All the organisms tested varied in their percentage susceptible to the extract of the plant leaves at different concentrations, except Klebsiella pneumonia which was 100% resistant. The Gram-positive organisms were more sensitive to the plant extract than the Gram-negative organisms. The phytochemical analysis reveals the present of flavonoids, saponins, tannins, cardiac glycosides terpenes and phlobatanins. Cotus afer leaves can be used for treatment of disease associated with tested organisms excluding K. pneumonia.