A series of 1-ethyl-6-fluoro-1,4-dihydro-4-oxo-7(1-piperazinyl)quinoline-3-carboxylic acid (norfloxacin) derivatives were prepared according to the principle of combinating bioactive substructures and tested for their activities against five plant pathogenic bacteria and three fungi in vitro. The preliminary bioassays indicated that almost all synthesized target compounds retained the antibacterial activities of norfloxacin and had some antifungal activities as carboxylic acid amide compounds. The activities of compounds 1 and 22 against Xanthomonas oryzae were better than norfloxacin and all tested compounds had better antibacterial activities as compared to the agricultural streptomycin sulfate (a commercial bactericide) against X. oryzae, Xanthomonas axonopodis and Erwinia aroideae. Additionally, compounds 2 and 20 displayed good antifungal activities against Rhizoctonia solani and their inhibition of growth reached 83% and 94% respectively at the concentration of 200mg/L.