Perfluoroalkanoyl peroxides (PFPO) were applyed to the surface modification of silk fabrics. The modification with PFPO proceeded under quite moderate conditions. PFPO reacted specifically with tyrosine residues of silk on the surface, and hydrophobic perfluoroalkyl groups were introduced to the surface effectively. The modification with PFPO influenced the wettability and electrostatic property of silk fabric. Based on the results of ESCA analysis, the amounts of fluorine atoms on the surface were less than those by the CF4 plasma modification. Moreover, the PFPO modification did not influence significantly the bulk properties, such as tensile stress and fabric hand. On the other hand, the PFPO modification lowered the light-resistance of silk. A possible reason was proposed based on a molecular orbital calculation for perfluoroalkyl-substituted tyrosine.