The nanocrystalline silicon films were prepared by pulsed laser ablation at the ambient pressures from 1 to 500 Pa of pure Ar gas. The x-ray diffraction spectrum indicates that the films are nanocrystalline, i.e. they are composed of Si nanoparticles. Scanning electron microscopy shows that with increasing gas pressur e, the average size of Si nanoparticles first increases and reaches its maximum (20nm) at 100Pa, and then decreases. The dynamics are analysed theoretically to explain the phenomenon. Furthermore, our result is compared with that in He gas.