Cotton fabric was treated using styrene/helium glow plasma at the atmospheric pressure. After the treatment, the substrate was found to turn into a highly hydrophobic material showing water drop disappearance time of 60 min and water contact angle of 133A degrees. The treatment was found to be durable even after vigorous washing. The effect of various parameters, such as discharge voltage and frequency, on fragmentation of styrene inside plasma zone was investigated using optical emission spectroscopy and GC-MS. The types of fragments formed in the plasma zone were correlated with the hydrophobicity of the substrate. The chemical nature of surface of the substrate was analyzed using SEM and Raman spectroscopy to elucidate the possible mechanism of plasma modification.