A methanol extract from yellow mustard seeds had antibacterial activity against Escherichia coli, Salmonella enteritidis, and Staphylococcus aureus. Two compounds with such activity were isolated from the extract. By instrumental analysis, the compounds were identified as 4-hydroxy-3-nitrophenylacetic and sinapic acids. Examination of the structure-activity relationship showed that the hydroxyl and nitro groups of the first compound were involved in the activity against all three species. The two methoxyl groups and the hydroxyl group in sinapic acid were effective against E. coli and all of the substituents of the benzene ring were effective against S. enteritidis. The presence of the propenoic group of the second compound was effective against S. aureus.