The authors have studied the microstructure of microcrystal air combustion products of aluminum nanopowder and the effect of magnetic field on their structure. It has been ascertained that two level filamentary crystals are the combustion products of loose aluminum nanopowders. At air combustion the hexagonal form monocrystals are formed in uniform magnetic field and the prolate form edged crystals of micron size are formed in the case of inhomogeneous magnetic field. The formation of stretched-out structure products can be explained by mass transfer of combustion products towards heat flows. The ordered hexagonal crystal structures are probably formed due to the oxidation products overheating under magnetic field action and the increase of mobility of their structural units in thermal wave of crystallization. According to the electron microscopy the formation of the edge crystals is possible at optimal size of combustion products particles.