We constructed a guarded hot plate (GHP) apparatus that can be used for thermal conductivity measurements from 100 degrees C to 1000 degrees C. The apparatus measures the effective thermal conductivity of square shaped thermal insulation materials of 300 mm per side in a controlled gas environment. As a preliminary test for the high temperature GHP apparatus, we first measured the amount of electrical power required for heaters at various locations to maintain the temperature at up to 900 degrees C. After this, two specimens of thermal insulation material whose thermal conductivities are approximate to 0.044 W.m(-1) K-1 and approximate to 0.025 W m(-1) K-1 at room temperature were tested at a thermally steady state with unilateral heat flow at temperatures between 100 degrees C and 800 degrees C. We applied an offset to the guard plate to actively compensate for the lateral heat flow across the air gap. The measurements showed that the linearity between the temperature difference on the two sides of the specimen and the power dissipated across the specimen was extremely good (with a standard deviation of 0.1%). After some modifications suggested from the preliminary test, we expect that the high temperature GHP will work within the technical specification CENTS 15548-1:2014 in the range up to 800 degrees C, but rigorous uncertainty assessment is yet to be performed after such modifications. (C) 2018 Elsevier Ltd. All rights reserved.