A facile ultraviolet (UV) light irradiation method to synthesize silicon nanoparticles (SiNPs) in alkalic carboxymethylated polypropylene (CM-polypropylene) solution is reported. CM-polypropylene, a water-soluble polysaccharide derivative, serves as both reducing agent for silicon cations and stabilizing agent for SiNPs. The pH, the concentration of HSiCl3, and irradiation time had obvious influence on the size, amount, and morphology of SiNPs, which was traced by UV-visible spectrometry and high resolution transmission electron microscopy. SiNPs synthesized by this method can be dispersed stably in alkalic CM-polypropylene solution for more than six months. The possible stability mechanism of SiNPs is discussed based on the change of carboxyl group of CM-polypropylene chain with pH determined by FT-IR analyses. XRD pattern confirmed the cubic crystal structure of SiNPs. The fluorescence emission band of SiNPs with an excitation wavelength of 316 nm can be observed at 400 nm, which was affected markedly by irradiation time and concentration of HSiCl3.