Abstract: Eighteen pyridonecarboxylic acids, characterized by a methyl group at the C₆-position instead of the usual florine atom, cyclopropyl at the N₁-position, substituted amino groups at the C₇-position, and some subtituted groups (hydrogen, chlorine, nitro and amino) at the C₈-position, were synthesized. The in vitro antibacterial activity of these compounds were tested, and the structure-activity relationships were also discussed. The results of the study showed that the activity of compounds 22 and 24 were less potent than that of ciprofloxacin against S.aureus, S.epidermidis, E.coli and P.aeruginosa, while they were 2～100 times more potent than ciprofloxacin against _{K.pneumoniae, S.marcescens, A.calcoacetous, E.aerogenes, S.typhi} and _{S.typhimurium}.