It is shown that second harmonic generation from a thin molecular film can be used for characterizing the crystallization and premelting kinetics of the crystallites in the film. The new observations provide a model understanding of the growth and crystallization mechanism in thin films made of molecules, such as pyridine, with very weak intermolecular interactions. The amorphous pyridine films, with thickness less than 100 nm, deposited at 82 K under UHV conditions on Ag(1 1 1) are found to crystallize via a diffusion-limited process at temperatures above 120 K, forming polycrystallites through a cylindrical-plate shape. This shape suggests that film thickness is an important factor in determining the crystallization mechanism. The boundary of the polycrystallites in the annealed pyridine film is found to crystallize or melt reversibly with decreasing/increasing temperature in a temperature range (80-135 K) much lower than the bulk melting temperature and the film desorption temperature.