Abstract: The thermal conductivity of mullite at the temperature of 150~500℃ was measured by guarded hot plate method, and the uncertainty sources were analyzed. The standard uncertainty components, combined standard uncertainty and expanded uncertainty were evaluated by GUM method which is a basic method for uncertainty evaluation in standard ISO/IEC 98-3:2008, and the influences of each measuring component on the measurement uncertainty were calculated. Additionally, the evaluated uncertainty was verified by Monte Carlo method (MCM). The results show that the relative expanded uncertainty (k=2) of thermal conductivity of the mullite was lower than 4%. The uniformity of temperature distribution in the plate plane dominated and had the greatest influence on the measurement results of thermal conductivity. The evaluation results at 150℃ were verified by MCM, and the results proved that the measurement uncertainty of thermal conductivity of mullite could be evaluated by GUM method.