It has recently been found that the thermal diffusivity of materials determined by means of a pulse method is strongly dependent on the specimen size, due to factors such as a strongly negative temperature-dependent diffusivity and the non-linearity of the detector response, etc. In polycrystalline (Sr,K)Zr-4(PO4)(6)(SrKZP) ceramic, which has a low and weak temperature-dependent thermal conductivity, it has been found the thermal diffusivity increases with increasing specimen thickness and decreases with increasing specimen temperature rise, Delta T. In the limiting of Delta T-->0, the SrKZP ceramic shows a thickness-dependent thermal diffusivity. The limiting diffusivity of the ceramic for different thicknesses can be corrected to a value of 7.84 x 10(-3)cm(2)s(-1) by taking the radial heat conduction effect into account. A reliable measurement of the thermal diffusivity of the SrKZP ceramic can be obtained by using a specimen with thickness not exceeding similar to 1 mm and keeping the specimen temperature rise to less than similar to 3 degrees C.