To evaluate the thermal properties of the constituent materials of the next generation of power modules, a precise evaluation method must be established, particularly for high thermal conductivity materials used in high temperature environments (i.e., over 200°C). In this study, the thermal conductivity of ceramics, silver sinter layers, copper substrates, and thermal interface materials (TIM) such as heat dissipation sheets/grease were evaluated using the improved longitudinal comparative heat flow method with a cartridge type sample, and then suitable criteria for improving measurement accuracy were investigated. Our results show that temperature uniformity in both upper and lower cartridges, as well as temperature differences between the upper and lower cartridges, were largely related to the accuracy of the thermal conductivity measurements. Therefore, the accuracy of the thermal conductivity measurements can be improved by controlling these heat transfer parameters in the evaluation method.