Fluorescence spectroscopy was used to investigate the influences of carbon nanotubes (CNTs) on the fluorescence of bovine serum albumin (BSA), the influences of CNTs on that of gatifloxacin (GFLX) , and the influences of GFLX on that of BSA with or without coexisting CNTs under imitated physiological condition. The experimental results demonstrate that both gatifloxacin and carbon nanotubes could quench the intrinsic fluorescence of BSA, and the quenching mechanism is mainly static quenching. The fluorescence quenching action of GFLX on BSA was weakened in the presence of CNTs. The fluorescence quenching data were analyzed according to Stern-Volmer equation and double-reciprocal Lineweaver-Burk equation. It was shown that the quenching constant (KSV) and the binding constant (K) are decreased with the concentration of carbon nanotubes increasing. The effects of coexisting CNTs on GFLX-BSA interactions were discussed to offer a reference for the studies on the action mechanism of CNTs or GFLX with albumins in vivo.