A new theoretical equation that describes the thermal conductivity of two-phase composite materials has been proposed. The Cheng-Vachon equation has been modified by introducing a new parameter named P-d max, permitting the new equation to describe the thermal conductivity of composite materials for a wide variety of filler shapes and states of dispersion. The new equation can describe the thermal conductivity of two-phase materials more accurately than any of the previous equations. Furthermore, this new equation will make it possible to evaluate the dispersion state of the discontinuous phase by measuring the thermal conductivity of the filled polymers or the polymer blends.