The interaction between 4-aminobenzenesulfonic acid and bovine serum albumin (BSA) had been studied by fluorescence spectroscopy. The binding constant (K=2.51×104) and binding points (n=0.998) of 4-aminobenzenesulfonic acid with BSA were determined at 298 K based on fluorescence quenching results. The thermodynamic parameters such as enthalpy change (ΔH), entropy change (ΔS) and Gibbs free-energy change (ΔG) for the reactions were also calculated according to the thermodynamic equations. The negative ΔH and ΔS values in case of 4-aminobenzenesulfonic acid-BSA complexes showed that van der Waals interactions and hydrogen bonds might play a major role in the binding of 4-aminobenzenesulfonic acid to BSA. The distance, r, between donor (BSA) and acceptor (4-aminobenzenesulfonic acid) was obtained according to the Förster’s theory of non-radiation energy transfer. The results of synchronous fluorescence spectra revealed that 4-aminobenzenesulfonic acid induced conformational change of BSA.