Abstract: Objective　To study the relationship between mutation of IgyrA/I gene of multi-drug resistant IAcinetobacter baumannii/I (MDR-ABA) and the resistance to ciprofloxacin. Methods　With IgyrA/I gene sequence of ABA as target sequence, mutation of IgyrA/I gene of 62 strains of ABA were analyzed by using PCR and DNA sequencing. Results　 Among 53 strains of ABA which were resistant to ciprofloxacin, 52 strains showed single point TCATTA mutation in the IgyrA/I gene at codon 83. Leading to amino acid substitution of SerLeu. Eight strains of ABA which were sensitive to ciprofloxacin and 1 strain of ABA which was intermediary to ciprofloxacin showed no mutation. Conclusion　The molecular mechanism of the resistant MDR-ABA clinically isolated was the mutation in IgyrA/I gene at codon 83.