Silica-coated magnetite nanoparticles were synthesized via sol-gel process, which comprises the hydrolysis and condensation of TEOS in ethanol/water solution with NH3 aq. as catalyst. Transmission electron microscope and vibrating sample magnetometer was applied to characterize the nanocomposite particles. Results reveal that the reaction parameters including volume ratio of alcohol/water, the concentrations of NH3 aq and TEOS play important roles of the formation of silica shell. The thickness of silica shell increased with TEOS concentration. The nanocomposite particles remained favorable superparamagnetism. The formation of silica shell mainly came from the aggregative growth of silica primary particles on the surface of magnetite nanoparticles.