The thermal behavior of power electronics devices has been a crucial design consideration, because it is closely related to the reliability and also the cost of the converter system. Unfortunately, the widely used thermal models based on lumps of thermal resistances and capacitances have their limits to correctly predict the device temperatures, especially when considering the thermal grease and heat sink attached to the power semiconductor devices. In this paper, frequency-domain approach is applied to the modeling of the thermal dynamics for power devices. The limits of the existing RC lump-based thermal networks are explained from a point of view of frequency domain. Based on the discovery, a more advanced thermal model developed in the frequency domain is proposed, which can be easily established by characterizing the slope variation from the bode diagram of the typically used Foster thermal network. The proposed model can be used to predict not only the internal temperature behaviors of the devices but also the behaviors of the heat flowing out of the devices. As a result, more correct estimation of device temperature can be achieved when considering the cooling conditions for the devices.