AbstractA hydrophobic modification of cotton fabric was demonstrated with atmospheric pressure plasma treatment with oxygen as the reactive gas. Oxygen plasma was determined to be capable of inducing hydrophobic modification of cotton fabric surface by utilizing the ageing effect. Upon ageing, the surface polarity was reversed and hydrophobic aliphatic hydrocarbons were formed, which was confirmed by Fourier Transform Infrared Spectroscopy. Surface hydrophobicity was quantified by the wetting area measurement. Wetted area of plasma-modified cotton was found to be strongly dependent on plasma-induced surface structures and the chemical composition on the fiber surface. Scanning electron microscopy revealed that physical morphological alteration was also a crucial factor that contributed to surface hydrophobicity. This work seeks to determine a controlled hydrophobic modification of textile materials through optimization of plasma process based on the Orthogonal Array Testing Strategy (OATS). Optimum process conditions were determined based on reduction of wetted area of plasma-modified cotton fabrics. Finally, hydrophobicity of plasma-modified cotton fabric was compared with conventional water repellency treatment.