TD-AL is a reference material for themal diffusivity measurements based on an alumina ceramic, Referceram AL1, which was originally developed as a common material for mechanical testing of advanced ceramics. This ceramic has also been studied as a possible reference material for measurement of themal expansion and Young’s modulus. In this study, the high-temperature stability of some themophysical properties such as themal diffusivity, themal expansion, and Young’s modulus has been evaluated. These properties were measured before and after heat-treatment in air for 50 hrs over a temperature range of 1200 to 1600°C. Heat-treatment at 1200°C resulted in no change in these properties. However, after heat-treatment at 1300°C or above, changes caused by grain growth in the ceramic were obsered. It is concluded that Referceram AL1 is stable and therefore suitable for use as a reference material below 1200°C.