The plaster mixed to loam and sandy soil from 4 to 12 percent by dry soil weight, and the compaction, permeability, CBR, unconfined compressive strength and freezing-thawing test were performed The results obtained are summarized as follows; 1. The coefficient of permeability reduced sharply at the plaster content of 4 percent, and in the CBR test, the swelling ratio reduced by the increment of plaster content. 2. The addition of plaster incre1sed the unconfined compressive strength by the cementing effect, and it was found that the optimum plaster content existed with the soil type, which showed the maximum strength 3. It was possible to enhance the unconfined compressive strength of the gypsum-lime-soil mixtures when the optimum content of plaster was mixed to the hydrated lime. 4. In case of s3ndy soil, the relative frost heave decreased with the mixture of plaster, however in loam soil, the relative frost heave began to incre1s~ at the plaster content of 12 percent than non-treated soil. Therefore the optimum, plaster content existed for protecting frost heave by the different soil type. 5. The above summarized results make it possible to expect the effects such as improvement of soil properties, decrement of permeability, increment of unconfined compressive strength, and protection of frost heave, etc, therefore, it is considered that it is possible to use plaster as sub-base materials of rq1d.