The fabrication of $Al_2O_3/Al$ composites by pressureless infiltration technique was made to investigate the effects of processing variables such as content of Mg, processing temperature and time on the infiltration behavior of molten Al and microstructure. When the pure Al was infiltrated into mixtures of Mg and $Al_2O_3$ powder, processing temperature required to spontaneous infiltration was decreased and critical processing temperature and Mg content were $700^{circ}C$ and 3wt% respectively. The content of Mg was found the most powerful variable for infiltration of molten Al. The infiltration ratio increased with Mg content and processing temperature, however the $Al_2O_3/Al$ composites which were fabricated by high Mg content and processing temperature resulted in non uniform dispersion of $Al_2O_3$ particles by excessive interfacial reaction. XRD pattern indicated that $MgAl_2O_4$ and AIN was observed at the interface of $Al_2O_3$ particles and in the Al matrix as reaction products.