Abstract: Mono-disperse spherical SiO2 were successfully synthesized by sol-gel method in the isopropanol-H2O-NH3 system. Tetraethylorthosilicate (TEOS) and ammonia were used as raw materials and catalyst, respectively. The as-prepared SiO2 powder was characterized by XRD, SEM, FT-IR and UV-VIS. The results indicated that the synthesized SiO2 powder is amorphous with regular mono-dispersed spherical feature, uniform size, smooth surface, good dispersity and narrow size distribution. There are broad absorption bands at 1126, 805 and 475 cm^-1 in FT-IR spectrum, which are attributed to the asymmetric stretching vibration, the symmetric stretching vibration and the bending vibration of Si-O-Si bond, respectively. The SiO2 powder has strong ability to absorb ultraviolet. The concentration of ammonia has notable influence on the size of ultrafine SiO2 particles and is the key factor of the hydrolysis of TEOS.