Surface modification of poly (ethylene terephthalate) (PET) fiber was investigated using low discharge polymerization technique. Argon gas plasma was used as an initiator on PET film and followed post copolymerization of acrylic acid vapor. The effect of modification was evaluated by contact angle of water, electro-resistance, IR-a.t.r. method, ESCA techniques and scannig electron microscopy. It was observed that the modification in the PET fiber was closely dependent on the gas pressure, treating time, current density, distance of substrate from gas inlet and reactor design. Generally, it seemed more effective to treat under the following conditions: 1) Plasma gas pressure is around 0.2 Torr and initiating time is 30∼45 seconds. 2) The more the high pressure, the more the long treating time of monomer gas is advantageous. 3) The higher current density in the low temperature plasma condition is required. 4) It is advantageous to avoid near of electrodes and center area between both electrodes for a setting position of substrate. In this work, the thickness of grafted thin film evaluated by ESCA techniques was 15${AA}$