Abstract: Modified silica hydrosol was prepared by water-based sol-gel method using methyltrimethoxysilane (MTMOS) as the precursor and hexadecyltrimethoxysilane (HDTMS) as a hydrophobic additive, in the presence of ammonia and the anionic surfactant sodium dodecyl benzenesulfonate (SDBS). Superhydrophobic cotton fabrics were successfully obtained by dip-coating the modified silica hydrosol. By controlling ammonia amount and surfactant concentration, the modified silica hydrosols with different particle sizes and PDIs were prepared. The effect of the SiO2 nanoparticle size and distribution on the water-repellent property of the treated cotton fabrics was discussed. The treated cotton fabrics were characterized by scanning electron microscopy (SEM), atomic force microscope (AFM), X-ray photoelectron spectroscopy (XPS). The results showed that the modified silica nanoparticles had been coated onto the cotton fiber surface, resulting in the lowered surface free energy and the increased surface roughness. The water-repellent property of the treated cotton fabrics were evaluated, and it was demonstrated that the water contact angle and water sliding angle of the treated fabric were respectively 152.1° for a 5µL droplet and 8° for a 15µL droplet, and the spray rating was 100, thus realizing superhydrophobicity. The physical and mechanical properties measurement of fabrics showed that there was a slight change after finishing except the permeability.