High-purity spherical silica nanoparticles have been successfully synthesized using powder quartz as silica sources in an improved sot-gel method. The effects of the modulus (molar ratio SiO7/Na2O) of sodium silicate and poly(ethylene glycol) (PEG) on the properties of silica nanoparticles were investigated. The results suggested that with the increase of modulus, the purity of silica nanoparticles increased. The higher purity (99.92 wt %) products can be obtained from the modulus of 3. Moreover, it was also found that the particle size, distribution, and dispersion significantly depend on the concentration of PEG. The silica nanoparticles with average size of 60 nm are well-dispersed and show a narrow size distribution when the PEG concentration is 2 wt %. The results suggest that it is feasible to prepare high-purity spherical silica nanoparticles using powder quartz as silica sources.