The translational diffusion coefficients of ketyl radicals produced by a hydrogen abstraction reaction in ethanol, methanol, and cyclohexane are investigated by the transient grating (TG) method, from liquid phase to the medium-density region in wide temperature ranges. It is found that the diffusion coefficient of the radical (D-r) is smaller than that of a stable molecule with a similar size (D-s) even under supercritical conditions. In the medium-density region, the values of D-r in alcohols are differently dependent on temperature over viscosity from those in cyclohexane. The activation energy of D-r was determined and compared with that of solvent viscosity. We found that D-p/D-s decreases with decreasing density in the medium-density region for cyclohexane. This tendency is explained in terms of the local density enhancement in the medium-density region. In alcohols, the density dependence of D-p/D-s is not significant.