The thermal rectification of a conductive thermal diode made up of two phase-change materials, whose thermal conductivities significantly change within a narrow interval of temperatures, is theoretically studied and optimized. This is done by deriving analytical expressions for the temperature profiles, heat fluxes and rectification factor. An optimal rectification factor of 60% is obtained for a thermal diode operating with terminals of VO2 and Polyethylene with a temperature difference of 250 K spanning the metal–insulator transition of both materials. It is shown that this high rectification of a conductive thermal diode could be maximized even more by increasing the thermal conductivity variations of both diode terminals. The obtained results can thus be useful to guide the development of phase-change materials capable of optimizing the rectification of conductive heat fluxes.