Methods for treatment of waterlogged archaeological materials, e. g. PEG, have many drawbacks such as slowness or complexity. Freeze-drying using Mannitol and PEG has been investigated as an alternative safe and simple method. The results are as follows: 1) waterlogged organic material should be treated at room temperature and the solution density necessary for dimensional stability should be investigated; 2) a new method using PEG 4000 after Mannitol treatment has been developed for remedying the drawbacks of the treatment; 3) the phenomenon of resisting shrinkage of the organic material by PEG and crystallization of Mannitol has been clarified by scanning electron microscope investigation. The Mannitol/PEG method in freeze-drying waterlogged organic material is much more useful than previous procedures because it maintains dimensional stability, requires short time, and is simple.