Moringa oleifera seed extract has been found to possesses antibacterial properties. In this research, the antibacterial property of the defatted seed extract (after oil removal) was investigated against 2 Gram-negative bacterial strains (Escherichia coli and Pseudomonas aeruginosa) and 2 Gram-positive bacterial strains (Bacillus subtilis and Staphylococcus aureus). Water and 1 Molar salt solution were used to prepare the defatted seed extracts. Susceptibility tests were carried out using the agar well diffusion method. The effect of increase in concentration of the seed extract was investigated and the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) were also determined. The results showed that the diameter of the inhibition zone ranged from 13 to 30 mm. The increase in the seed extracts showed a corresponding increase in the inhibition of the bacterial strains. The salt extract of the seed showed higher increase in antibacterial activity against the bacterial strains when compared with the water seed extract. The MIC values of defatted water seed extract were 0.39, 0.39, 0.78 and 0.78 mg/ml, and those of the salt solution seed extract were 0.78, 0.78, 0.78 and 0.78 mg/ml for E. coli, P. aeruginosa, B. subtilis and S. aureus, respectively. The mode of action of the seed extracts showed that it was strongly bactericidal against E. coli and P. aeruginosa and bacteriostatic against B. subtilis and S. aureus.