To determinate the nucleation region of Si nanoparticles formed in gas phase, the single crystalline Si target with high resistivity was ablated by a XeCl excimer laser in pure Ar gas under the ambient pressure of 10Pa, and the nanocrystalline Si films were systemically deposited on pieces of glass or single crystalline (111) Si substrates lined up at a distance of 2.0cm under the plasma. The Raman and X-ray diffraction spectra, scanning electron microscope and atomic force microscope images of the films show that Si nanoparticles were formed on the pieces placed at horizontal distances 0.5 to 2.8cm from the target, the average size of Si nanoparticles monotonically decreased with the distance increasing. The region that Si nanoparticles form in gas phase was estimated on the basis of the PLA dynamics.