Abstract: Nano-SiO₂ is modified by silane coupling agents with different carbon chain lengths (methyltrimethoxysilane, propyltrimethoxysilane, octyltrimethoxysilane and cetyltrimethoxysilane). The chemical structure and grafting efficiency of the nano-SiO₂ before and after modification are studied by FTIR, XPS, XRD and TGA. The particle size distribution and micro morphology of nano-SiO₂ particles are studied by laser particle size measurement and TEM. The dispersion and hydrophilic and hydrophobic properties of the nano-SiO₂ particles in water are observed by the sedimentation experiment and contact angle. The results show that the silane coupling agents are grafted on the surface of nano-SiO₂ particles. With the increase of carbon chain length of the silane coupling agent, the average particle size of the modified nano-SiO₂ first decreases and then rises, and the agglomeration phenomenon weakens. The contact angle of the nano-SiO₂ particles modified by cetyltrimethoxysilane increases from 11.7° to 108.2°, the hydrophilic property disappears and the hydrophobic property appears.