A partition coefficient (lambda) for Ra in gypsum was determined from coprecipitation experiment using oversaturation method. The lambda-value derived for Ra was: lambda (Ra) = 0.32 +/- 0.15. Saturation state was estimated with the initial concentration of Ca and SO (4) (2-) in an experiment of the present study. Saturation index was calculated to be (0.49 +/- 0.02). This value was similar to those corresponds to analogous case where slow precipitation rate was kept in coprecipitation experiment for Sr in gypsum. Therefore, derived lambda-value is thought to describe partition of Ra and Ca in solid phase and solution under an equilibrium condition without the effect of kinetic of precipitation. Determined lambda-value was compared with those of other alkaline elements in sulfate minerals. The derived lambda-value is smaller than that of Ra in barite and is similar to those of Sr in gypsum. This trend agrees with mechanical understanding for the size effect against partition coefficient.