The thermal conductivity of ZnO with different particle sizes (micrometer, submicrometer, and nanometer) was measured using the laser flash technique. As the "green" samples were heated from room temperature to 600degreesC (and 1000degreesC) and then cooled down to room temperature, the thermal conductivity was measured in situ. A model for interparticle neck growth was developed based on mass transfer to the neck region of a powder as a result of known temperature. By combining this model with a three-dimensional numerical code, the thermal conductivity of ZnO was calculated. Excellent agreement between the theoretical calculation and experimental data was found.