Thin-film coating on cotton by the admicellar polymerization process was investigated. In this work, styrene was used as the monomer to coat styrene on cotton. The effects of surfactant, styrene, initiator, and electrolyte concentrations on the polymerization process were determined. The polystyrene film formed was characterized by SEM, FTIR, and GPC. The increase in the hydrophobicity of the treated cotton surface was determined by the drop test. Results show that polystyrene thin film was successfully formed on cotton, resulting in cotton that can resist wetting by a water droplet for longer than 30 min.