A miscible PS/PPO blend was prepared and its miscibility was confirmed by d.s.c. It was found that the miscibility of polymer blends could be evaluated by the measurement of their thermal diffusivity. The conductivity in the liquid state was smaller than that estimated from a linear-relation with the PPO content, but in the solid state, the conductivity was larger than the estimated value. This phenomenon was considered to occur because the strong interactions of polymer chains in the solid state increased the density (negative excess volume), resulting in the increase of the thermal conductivity of the blend. Further, all of Filippov, NEL and power law equations were found to be sufficiently adaptable to the experimentally determined thermal conductivities of PS/PPO blend in the liquid state, but none of them could be adapted to that in the glass state.