Ni 3 Al intermetallic compound was extensively investigated due to special properties that show in high temperature. Combustion synthesis is one of the new methods for producing the intermetallic. High rate of reaction, high-purity of products and low energy consumption are main characteristics of the combustion synthesis process. Because of the high rate of combustion synthesis reaction, understanding kinetics and mechanism of the reaction for process controlling and producing of product with required properties are very important. Therefore in this study, the mechanism of Ni 3 Al formation from powder mixture of nickel and aluminum with stoichiometric ratio (Ni/Al=3) was investigated in SHS process. Path and mechanism of Ni 3 Al reaction were proposed the effective parameters on the process were investigated by using microscopic and phase analysis studies. After the sample ignition by oxyacetylene flame and combustion wave propagation the sample quenched in liquid nitrogen to stop the reaction. Evolution of reactant conversion to product was studied as microstructural in sample quenched by scanning electron microscopic (SEM) equipped to energy dispersive spectroscopy (EDS). Also layer by layer phase analysis done by X-ray diffraction (XRD). For evaluation of proposed mechanism accuracy, supplementary examination of thermal analysis (STA) was preformed on powder mixture with different condition. It was observed that reaction of the formation of Ni 3 Al compound during combustion synthesis was preformed in two mode solid-liquid and solid-solid based on dissolution and precipitation mechanism. The path of solid-liquid reaction includes dissolution of nickel in liquid aluminum and precipitation of Ni 3 Al from supersaturated Al(Ni) liquid and also formation of NiAl compound. On the other hand in solid-solid reaction, path of reaction includes the formation of Ni(Al) solid solution and then formation of Ni 3 Al from supersaturated solid solution. With increasing interface of powder particle with each other combustion synthesis reaction was preformed in solid-solid mode, whereas after 40 hour milling of powder mixture, starting reaction temperature decreased from 620?C in solid-liquid mode to 327?C and reaction doing in solid-solid mode. In both paths of reactions, diffusion control processes are more important in synthesis of the formation reaction Ni 3 Al compound. Keywords: Ni 3 Al, Combustion synthesis, Reaction mechanism, Milling