Abstract: A new method to improve the mechanical properties of nano porous silica films is reported. The thin films are prepared using the base/acid two step catalytic sol gel process and the dip coating method, and then annealed in the mixed gas of ammonia and water vapor. The films are characterized with TEM, AFM, FTIR, spectrophotometer and ellipsometery, respectively. The experimental results show that the two step catalysis remarkably strengthens the films, and the scratch resistance of the silica films is further improved after treatment in the mixed gas. The increase in strength is attributed to the cross linking of silica particles by the two step catalysis and the mixed gas treatment. And the films still have low refractive index(1.24) as well as much high transmittance over 99.5%.