In order to assess the performance of drowning-out crystallization using a T-mixer, nucleation and supersaturation were studied. The particle size was changed considerably with the solvent fraction and the feed rate, which were the main parameters controlling the supersaturation. At S=1.7, the boundary point between homogeneous and heterogeneous nucleations was found from a log-log plot of supersaturation rate versus maximum supersaturation. Drowning-out crystallization using a T-mixer could easily generate high supersaturations of up to 50, which were adjusted by the feed rate and the ratio of solvent to antisolvent. Nano- and micron-sized particles can be prepared by drowning-out crystallization using a T-mixer.