Abstract: It is found that retardation coefficient, R d, of 85 Sr in unsaturated loess medium is not constant and increases with water content, θ, according to analyzing and fitting to the results obtained from the field tracer migration and laboratory simulation tests. The change tendency of retardation coefficient with water content in loess is in contrary to the results calculated from conventional formula. It is shown that it can't be used for assessing safety of a specific site derived R d using distribution coefficient, k d, measured by batch method in laboratory. Moreover, application range of conventional formula and the method of measuring k d are worth studying._