The buffer gas pressure is one of the most important parameters which influence the size of nanoparticles in the synthesized process by pulse laser ablation (PLA). Usually speaking,the size of nanoparticles increases with the increase of buffer gas pressure. In this report,according to the scanning electron microscopy (SEM) measurements and corresponding statistic results for particles size of about 10 nm,it can be obtained that,the size of silicon nanoparticles decreases with the increasing of the gas pressure in the range of 50-100 Pa,and a relative expression between the gas pressure and the size of the silicon nanoparticles can be given,which describes the experimental phenomenon properly. The photoluminescent (PL) properties of silicon nanoparticles are investigated by photoluminescence spectroscopy with a wavelength of 280 nm at room temperature,and the violet and blue emissions are gained. A double peak structure at 372 nm and 445 nm appears in the sample synthesized by the gas pressure of 50 Pa. While the gas pressure increases to 70 Pa or 100 Pa,the peak in blue region disappears and the peak in violet region shifts to 379 nm and 393 nm corresponding to the samples synthesized by gas pressure of 70 Pa and 100 Pa respectively. These photoluminescent emissions are attributed to the surface state effect from the oxidized layer at the surface of the silicon nanoparticles.