Silicon carbide (SiC) power device is attracting attention as a candidate for future power device, since they can be operated at higher current density and higher switching speed than conventional silicon (Si) power device. A power module packaging technology for SiC power device should have both low thermal resistance and low inductance to bring out a potential ability of the device. In general, high-speed with high-current switching causes huge surge noise due to the parasitic inductance of the module. We have introduced a double-stacked silicon nitride active metal brazed copper (SiN-AMC) substrate as a circuit board which has five-layer (Cu/SiN/Cu/SiN/Cu) structure by bonding two SiN-AMC substrates. The parasitic inductance of the module is reduced by minimizing the loop area of the current path using a internal conductor layers of the double-stacked SiN-AMC substrate as a current return path. However, it is predicted that the double-stacked SiN-AMC substrate power module has higher thermal resistance compared with the conventional power module which has single SiN-AMC substrate. In this paper, the thermal resistance of double stacked SiN-AMC was measured by transient thermal analysis method and investigated the magnitude of the effect of SiN thickness, Cu thickness and SiN thermal conductivity. The appearance of the test vehicle is shown in Fig. 1. Each type of the test vehicle which is composed of an SiC-Schottky barrier diode (SiC-SBD), one (or two) SiN-AMC(s) and an Al baseplate were fabricated. The specifications of the SiN-AMC substrate to be compared are shown in Table 1. The thermal resistance of each test vehicle was measured by transient thermal analysis method and the thermal resistance of the SiN-AMC substrate part was extracted. The measurement results of thermal resistance at SiN-AMC substrate part are shown in Fig. 2. In general, it is considered that increasing the thermal conductivity of a portion with a relatively low thermal conductivity (in this case it is SiN) in the module or thinning the total thickness of the SiN-AMC substrate leads to a reduction in thermal resistance. However, in case the SiC die size is 3 mm square, we found that the thickness of the Cu film is more effective than the thermal conductivity of SiN in order to reduce the thermal resistance of the module. Thermal resistance of double stacked SiN-AMC with 0.3 mm-thick Cu film is lower than the thermal resistance of single SiN-AMC with 0.1 mm-thick Cu film. The effect of changing Cu film thickness is higher than changing thermal conductivity of SiN from 90 W/mK to 140 W/mK. If Cu film of the SiN-AMC substrate is thick the thermal resistance is reduced, since the heat is diffused in the in-plane direction and the heat-flow area is increased. These results will help for optimizing the power module circuit board which has both low inductance and low thermal resistance.