Abstract: Based on the population balance model and ΔL law, a crystal growth rate equation was deduced, and an apparatus was built up and used to measure the on-line dynamic transmittance and concentration for the batch cooling crystallization process. The KNO3 in water was tested and its linear growth rate was also calculated with dynamic transmittance and concentration. When crystal growth was dominant in the later stage of crystallization, the KNO3 crystal growth kinetic parameters (kg and g) in the empirical model were obtained by correlating its linear growth rate with the relative supersaturation,and agreed well with those in the literature.For the spontaneous crystallization process, the nucleation and crystal growth stages could be identified qualitatively with the dynamic transmittance, supersaturation and the deduced linear growth rate equation. The growth rate equation and its solution in this paper would be further used for mechanism demonstrations and/or applications of the batch crystallization.