We focused on the surface reinforcement of Al casting alloys with Ni-Al intermetallic compounds by in-situ combustion reaction to improve the surface properties of Al casting components. Microstructure and phase formation behavior of Ni-Al based intermetallic compounds synthesized by combustion reaction were investigated in terms of thermal and phase analysis using scanning electron microscope(SEM) equipped with energy dispersive x-ray spectrometer (EDS) and x-ray diffractometer(XRD) in Ni-Al intermetallic compounds. Three kinds of nickel aluminides, NiAl(3), NiAl and Ni(3)Al, were synthesized by emission heat from the Al molten metal in order to form a coating layer of intermetallic phase simultaneously on the solidifed Al alloy surface. The synthesized shapes and microstructures of nickel aluminides were varied by casting temperature, Si contents, and the mixing ratio of elemental powders. The synthesized reaction products formed in nickel aluminides were observed to be different depending on the mixing ratio of elemental powders. The reaction layer of about 25 mu m thickness was formed at the interface, and it mainly consisted of NiAl(3) phase by the reaction between liquid molten Al alloy and solid Ni powders in green compact. With this information, we successfully produced a coating layer of Ni(3)Al intermetallic compound onto the casting Al alloy surface using molten metal heat without any additional process. These findings led us to conclude that a near-net shaped nickel aluminide coating layer can be formed using this unique process.