The laser flash method is generally acknowledged as a standard and most popular method to measure thermal diffusivity of solid materials above room temperature. Since surface of optically nontransparent and dark materials can be directly heated by the laser pulse and measured without black coating, we selected a grade of isotropic graphite and characterized homogeneity and stability as a candidate reference material. We proposed a new calibration procedure of the laser flash instruments using a set of specimens of same thermal diffusivity with different thicknesses. The specimen sets with SI traceable thermal diffusivity value will be supplied soon.