In crystallization processes, measurements and estimations of nucleation in a stirred vessel are important in the quality control of the crystal product. However, primary nucleation in a crystallizer is a very complicated and sensitive phenomenon, so much that obstacles such as impellers and temperature sensors in the crystallizer affect primary nucleation. The effects of the operation parameters on primary nucleation have not been well characterized. Herein, we attempted to establish a non-intrusive on-line measurement method for the initial dynamic behavior of a series of primary nucleation based on the detection of laser light scattering induced by small particles. Firstly, properties of laser-scattered light by particles suspended in a stirred vessel were investigated under various size and mass conditions for polystyrene particles. Secondly, with these results for model particles, we attempted to apply the measurement method to the quantification of the initial dynamics of a series of primary nucleation of batch cooling crystallization of glycine. The results show that there are cases in which the super-saturation of solution increases in spite of increases in crystal precipitation mass.