Under contract from the National Institute of Standards and Technology, we have developed a prototype design for a miniature guarded hot plate apparatus that can be used to determine the thermal resistance of very small specimens (1 to 3 cm square) of thermal insulation materials at temperatures from -40 °C to +100 °C, with the capability to carry out measurements in air, selected gases, or vacuum. The overall design approach for this apparatus has been published previously. In the present paper, we provide details concerning the thermal design of the apparatus, including discussions of heater designs to promote temperature uniformity of the hot plate, guard designs to control extraneous heat gains and losses, material selection for the components of the apparatus, and techniques for temperature measurements.