The understanding of the reaction kinetics in organic molecular crystals is very important to design stimuli-responsive organic crystals that exhibit desired functions at arbitrary times. However, there are cooperative structural change processes involving multiple molecules due to intermolecular interactions, which makes the classical kinetic analysis difficult. The Johnson–Mehl–Avrami-Kolmogorov equation quantitatively evaluates cooperative nature of reactions, crucial for understanding kinetics in the crystalline state. However, it lacks insights into elementary reaction processes. Hence, the extended Finke-Watzky model, we have developed, is introduced for more detailed analysis of photoreactions. This model provides insights into elementary reaction processes, offering dynamic quantum yields of photoreactions. Furthermore, the progressive behavior of photoreactions on optical length scale is introduced. While photodimerization in 9-methylanthracene crystals occurs homogeneously on optical length scale, photopolymerization in 2,5-distyrylpyradine crystals exhibits edge-to-center propagation, influenced by cooperative reactions and surface effects. These findings enhance understanding of photochemical reaction kinetics in crystalline materials.