Grain size has a significant influence on the performances of materials. Cooling rate is a key process parameter for controlling the size of crystal grain. Real-time observations of crystallization process on an atomic scale under different cooling rates are helpful for an in-depth understanding of this scientific issue. However, it is very difficult to observe directly the crystallization process on an atomic scale because it is small in size and fast in motion. Over last decades, colloidal suspension has attracted many researches attention as a model system of condensed matter to investigate phase transition kinetics at a particle scale level because colloidal particles are micrometer-sized and their thermal motions can be directly visualized and measured with an optical microscope. Thermo-sensitive poly-N-isopropylacrylamide (PNIPAM) colloidal suspension is one of the model systems and its phase transition can be easily controlled by temperature. In this paper, the PNIPAM colloidal system is used to make the real-time observation of the influence of the cooling rate on crystal grain size. Firstly, the crystal nucleation and growth process of PNIPAM colloidal suspension at a cooling rate of 30.0 ℃/h is observed with a high-resolution transmission microscope. It is found that liquid-solid phase transition of the PNIPAM colloidal suspension begins from a sudden transient nucleation, followed by a rapid grain growth as temperature decreases. The variation of crystal phase fraction with temperature undergoes three stages: slow, rapid and slow. In the initial stage, nuclei are limited and the growth driving force is low, therefore the crystal phase fraction changes slowly. In the middle stage, as temperature decreases, the growth driving force further increases and the crystal phase fraction increases rapidly. In the final stage, the crystal grains begin to adjoin with each other and the left liquid volume becomes less and less, so the crystal phase fraction increases in a slow mode again. Secondly, the PNIPAM colloidal crystal under different cooling rates from 0.5 ℃/h to 30.0 ℃/h is observed with Bragg diffraction technique. The grain size of PNIPAM crystal is also measured. It is found that the size of PNIPAM colloidal crystal grain decreases with the increase of cooling rate and the relationship between the grain size and the cooling rate obeys a power-law formula, which is also used to well describe the effect of cooling rate on grain size in metallic system. This suggests that the crystallization behavior of PNIPAM colloidal system under continuous cooling is similar to those of metallic systems. However, the fitted power-law pre-factor of PNIPAM colloidal system is very different from those of the metallic systems because the sizes and motions of PNIPAM particles are much larger and slower than those of atoms, respectively.