The effects of the specimen temperature of low-density polyethylene (LDPE) in O-2 plasma treatment were studied to enhance the amount of hydrophilic functional groups introduced and to reduce the aging effect. The specimen temperature was varied from 25degreesC to 100degreesC. The smallest water contact angle was obtained with the 45degreesC specimen and the largest amount of hydrophilic functional groups was introduced with the 100degreesC specimen, as determined by X-ray photoelectron spectroscopy (XPS). Therefore, a two-step plasma treatment with two different specimen temperatures, i.e. 100degreesC followed by 45degreesC, decreased the water contact angle and reduced the aging effect. It appears that the hydrophilic functional groups introduced were located at the specimen surface (about 0.5 nm) at low temperature and that the aging effect was reduced due to the hydrophilic functional groups formed inside (0.5-8 nm) at high specimen temperature. The aging rate and the diffusion coefficient were also estimated, depending on the specimen temperature, using the experimental aging data.