To crystallize ammonium sulfate (AS) of a coarse fraction of 1 to 3 mm size, a semibatch evaporative crystallization mode was applied. The crystal size was controlled by supersaturation. The induction time and supersaturation were analyzed in seeded and unseeded semibatch crystallizations. The interfacial tension was obtained by nucleation experiments. Experimental results indicated that supersaturation was strongly dependent on the metastability of the system and had a strong influence on the crystal growth. This result will contribute to a better understanding of the control of the particle size of AS by supersaturation. Coarse AS crystals can be obtained at supersaturation of 0.02-0.06 in seeded semibatch evaporation crystallization.