Morphology of polyethylene (PE)/polystyrene (PS) blends has been investigated by measuring their thermal conductivity, which is analyzed with the Okamoto-Ishida equation. The results of the thermal conductivity measurement suggests that the PE/PS blend system can be classified into three regions in terms of morphology. The morphology of the PE/PS blends in each region is investigated by electron microscopy, microthermal analysis, and a selective extraction technique. The observed morphology is consistent with the suggestions from the thermal conductivity of the blends. The phase inversion points and co-continuous region of the PE/PS blend system are determined by applying the Okanioto-Ishida equation to the thermal conductivity of this blend system. It is concluded that the Okamoto-Ishida equation provides a nondestructive evaluation method for the morphology of polymer blends.