Nanocrystalline silicon powder was prepared by decomposition of SiH4 gas using an excimer laser beam, The laser was operated at 193 nm, a repetition rate of 10-70 Hz, and a pulse duration of 24 ns. The base vacuum in the chamber was 10(-6) Torr and the pressure during deposition was 1-5 Torr. Si particles were formed in the gas phase and deposited on Si(100) and also on SiO2 substrates. The microstructure of the Si powder was studied using analytical and high-resolution transmission electron microscopy. The photoluminescence of the powder was measured in wavelengths of 400-900 nm using excitation light of 488 nm and 330 nm. The powder was found to consist of nanocrystalline circular grains. The average size and size distribution of the grains were found to be strongly dependent on the pressure and flow rate of the gas mixture and on the repetition rate of the laser beam. Grains as small as 5 nm with a narrow size distribution were obtained. Photoluminescence (PL) emission was observed at room temperature from the Si powders in the following wavelengths : 610-670 nm, 510-550 nm, and 430-510 nm. The width and position of the PL peak were found to depend on the size distribution and average size of the Si grains.