This study was conducted to investigate the penetration of PEG for conservation treatment conditions of archaeological waterlogged wood. The weight percent gain was examined, depending on the solvent of PEG, concentration and treatment period and temperature of treatment solutions. The penetration of PEG in the cell lumina of treatment woods was observed by scanning electron microscope. The results showed that the type of solvents had no influence on PEG penetration. In the concentration of the PEG treatment solutions, the weight percent gains (WPGs) were increased with increase in concentration of PEG. In terms of the period of the soaking treatment, a maximum WPGs were obtained for only 20 days. There was no distinctive difference in the WPGs by difference the temperature of the treatment. In conclusion, in the conservation of small size of archaeological waterlogged wood, it is confirmed that optimal solvent type and treatment period of PEG are water and 20 days, respectively.