The thermal conductivity of Lucalox, a high‐purity form of aluminum oxide, has been measured over the temperature range 870–1400°C. The technique employed is a novel one using a lithium‐filled heat pipe in series with a gas gap variable heat conductance unit as a heat source. The procedure enables one to measure heat flux through the sample directly. The measurements indicate that the thermal conductivity of high‐purity aluminum oxide has a minimum value of about 0.05 W/cm°C at 950°C and increases with higher temperatures to values in the range of 0.3 W/cm°C at 1400°C.