Aluminum matrix composites have greatly been used in automotive, military and aerospace industries because of their high specific strength, good wear resistance, low coefficient of thermal expansion, and high oxidation resistance. One of the methods for fabricating aluminum matrix composites is in situ method, in which reinforcing phase is formed in matrix phase via chemical reactions between elements or between elements and other compounds. In this research, mechanically activated aluminum sulfate was injected into molten pure aluminum to produce in situ aluminum-alumina (p) composite. Mechanical activation of aluminum sulfate reduced its decomposition temperature from 950 ?C to aluminum melting point, which it was attributed to decrease of crystallite size, increase of lattice strain and specific surface area. Kinetics of thermal decomposition of aluminum sulfate was studied by thermogravimetric data under non-isothermal conditions. Although mechanism of decomposition reaction of aluminum sulfate was not changed due to mechanical activation, activation energy value was decreased with increasing of activation time. Aluminum sulfate was decomposed in melt to alumina and SO 3 , which later one left the melt without changing its composition. Scanning electron microscopy (SEM) was used to evaluate sizes and distribution of alumina particles in composite samples. It was found that distribution of alumina particles was improved with increasing of the rotation speed of impeller, and alumina particles size was almost one micron. Mechanical properties of fabricated composites were evaluated by hardness, wear, and tension tests. Weight loss of samples after 100 meters wear under 4 Newton load was reduced from 10.1 milligrams for non-composite specimen to 5.0 milligrams for composite one. Yield stress and ultimate tensile strength of specimens were increased, but their elongations were decreased with increased alumina percentages.