Powder compacts consisting of Al, Al2O3, and ZrO2 were heated by microwave radiation. Tracing the phase evolution during reaction bonding revealed the reaction mechanism. In the case of conventional heating, the compacts expanded slightly at temperatures of <700 degrees C due to Al surface oxidation and expanded sharply at temperatures greater than 700 degrees C as oxidation proceeded from the surface to the interior. Then, the compacts shrank at 1550 degrees C due to sintering. For the case of microwave heating, the compacts expanded at temperatures of <550 degrees C due to the formation of Al3Zr. This Al3Zr formation was caused by the preferential heating of ZrO2 relative to Al and Al2O3 by microwave radiation. Then, Al3Zr was oxidized to form Al2O3 and ZrO2 at temperatures of >1000 degrees C. Finally, the compacts shrank at 1550 degrees C due to sintering, similarly to conventional sintering.