The knowledge of crystallization kinetics, especially crystal nucleation, is crucial for crystallization process design and control. In the present paper, the primary nucleation kinetics of benzoic acid in water-ethanol solution was estimated from induction time measurements, where focused beam reflectance measurement was utilized to detect the onset point of nucleation. As a result, the induction times have been found to decrease with increasing mass fraction of water under the same supersaturation. These data were then used to determine the interfacial energy together with critical size and activation energy for nucleation based on classical nucleation theory. The resulting interfacial energy differed from 1.83 to 6.46 mJ/m(2) at various solvent compositions. Also, it has been demonstrated that the interfacial energy increased with increasing water mass fraction, which finally caused the increase of the critical size and activation energy for nucleation, suggesting significant effects of water mass fraction on nucleation kinetics.