Better thermal management is a key enabler of higher power density in traction inverter power modules. For the first time, we successfully fabricated and tested a microchannel with a three-dimensional (3D) manifold cooler (MMC) using aluminum nitride (AlN)-based directed bonded copper (DBC) substrates. The microchannels (width ∼300 μm and height ∼450 μm) and 3D manifold fluidic passages (width ∼300 μm and height ∼600 μm) were fabricated in two DBC substrates using the femtosecond laser and subsequently bonded using transition liquid phase (TLP) bonding. In this study, the hydraulic and thermal performance of the 3D MMC is measured and validated with numerical simulation. The proposed 3D MMC is capable of removing heat at 600 W/cm2 with a 10 kPa pressured drop at the thermal thermal resistance of 0.2 cm2 K/W. The optimized designs via geometric and layout rearrangement were conducted, which indicates the pressure drop can be further reduced by 10× while maintaining the same thermal performance.