To obtain strong hydrophobic property of silica，on the basis of traditional modification process with silane coupling agent stearic acid was introduced as complex modifier to modify the nano-silica powder. The modified silica particles were characterized with Fourier transform infrared analysis（FTIR），X-ray photoelectron spectroscopy（XPS），contact angle tests and sedimentation tests. The complex modification mechanism was also discussed. The results showed that amino group（—NH2）was grafted successfully onto the surface of nano-silica particles by modification with silane coupling agent. After the second modification with stearic acid，amide bond（—CONH—）was formed by amino group（—NH2）and carboxyl group（—COOH）of stearic acid. As a result，the —（CH2）3COHN—（CH2）16CH3 group was obtained on the surface of nano-silica particles，which had excellent hydrophobic property. Because of the complex modification，the stearic acid molecules were grafted on the surface of nano-silica particles by chemical bonds，and the contact angle could reach 140°.