This article presents thermal diffusivity (D) measurement by flash radiometry for the polymer blend of polycaprolactone (PCI;) and poly(bisphenol A carbonate) (PC) with a lower critical solution temperature (LCST) phase diagram. The dependence of D on temperature is significantly changed by annealing above the cloud point. D is largely increased by the growth of PCL crystallite in the blends. In the miscible state, D is smaller than that predicted by the rule of mixtures, whereas the dependence of D on the weight fraction of PC for the immiscible (phase-separated) state is well expressed by the rule of mixtures.