Surface modification of poly(lactic acid) (PLA) film surface by Ar-plasma was investigated by contact angle measurements and XPS in order to answer the following two questions. (1) Could the Ar-plasma modify the PLA film surfaces? (2) What chemical reactions occurred on the film surfaces during the Ar-plasma treatment? The Ar-plasma treatment did not lead to hydrophilic modification of the PLA film surface, but to degradation reactions of the PLA film. Poor modification may be due to instability of the carbon radicals formed from C-O bond scission in the PLA chains by the Ar-plasma.