We performe dynamics and thermodynamics analysis on the formation of nanodiamonds (NDs) produced by pulsed-laser ablation (PLA). The analysis on physical and chemical difference arise from two PLA processes based on laser parameters and synthesize methods are carried out,and the equilibrium size is also discussed upon thermodynamics factor as well as the growth velocity,growth time,and the final size of NDs are calculated using the Wilson-Frenkel growth law. The result shows that the radius and density of plasma plume determine the cooling velocity,and the growth time correlates with the cooling velocity when the temperature and pressure are high enough to meet the conditions of the stable growth of NDs. The calculation on the lowest free energy also indicates the formation of NDs is reasonable. Theoretical calculations are in good agreement with experiment results and give a well explanation on the formation of ultrafine NDs.