Nucleation and growth kinetics of ammonium sulfate are determined using a 1.5-L cooling batch seeded crystallizer, a rigorous mathematical model of the process, and a nonlinear optimization program. Supersaturation and crystal size distribution are employed as the major outputs of the model in the optimization objective function. The former is estimated by an online density meter, and the latter is measured by a Malvern Mastersizer. The size-independent growth kinetic is shown to lack the necessary parameters to accurately fit the experimental data. The information gathered from a large number of measurements in this study indicates that ammonium sulfate possesses a size-dependent growth kinetic. It is shown that a second-order size-dependent growth model reasonably describes the experimental measurements, while a combination of a second-order size dependence for small and medium crystals and a slightly lower dependence (order of 1.85) for large crystals results in the least error between experimental and simulated results.