The silica suspension was prepared by a sol gel method using tetraethylorthosilicate (TEOS) as precursor and ethanol as solvent. The stability of the sol system was observed by particle size test. The particle size maintained at about 6.5 nm for 4 months. The quartz substrates were coated by a dipping method and heat treated at 200 ℃ and 800 ℃ in a muffle burner. The film transmittance curve moves to short wavelength direction by about 210 nm, and both the transmittance peaks are more than 99.8%. Therefore, the film’ optical properties are excellent. The stabilities of film heat treated at different temperatures were researched in different humidity environments. The transmittances of the films heat treated at 200 ℃ are more than 99.9% and the wavelength of film transmittance peak decreases by about 150 nm. The transmittances of the films heat treated at 800 ℃ reduces by 0.3% and the wavelength of film transmittance peak maintain at 600 nm.