Ti 5 Si 3 intermetallic compound have several advantageous such as high melting point, high hardness, relatively low density and high temperature oxidation resistance especially in temperatures lower than 1000 0 C. Monolithic Ti 5 Si 3 has been fabricated by various methods including: arc melting of Ti and Si, rhock assisted sintering, reaction sintering and self propagating combustion synthesis. The final product in all of these processes was brittle and had low fracture toughness. It should be noted that low ductility and fracture toughness of this compound is related to low symmetry in crystal structure and highly covalent bonding that increase the peierls stress. Fracture toughness of Ti 5 Si 3 can be overcome by decreasing of grain size to nanometric scale, addition of third element and uniform distribution of fine stable reinforcements in the matrix. It is reported that Al 2 O 3 is a good reinforcement candidate in the intermetallic matrix composite. In fact desirable properties of Al 2 O 3 such as low density, high specific strength and high modulus can provide stiff ceramic inclusions into and intermetallic matrix such as Ti 5 Si 3 . In this study, Nanostructured Ti 5 Si 3 was synthesized by mechanical alloying of Ti 62.5 Si 37.5 powder mixture and the effect of Nb was investigated on the formation mechanism and thermal stability of this compound. Miedema model was used to predict first phase formed in the mechanical alloying of Ti 62.5 Si 37.5 . Afterwards Ti 5 Si 3 -Al 2 O 3 nanocomposites with different percentage of reinforcing phase were synthesized and the effect of diluents was studied on the formation mechanism. Nanostructured Ti 5 Si 3 and Ti 5 Si 3 -Al 2 O 3 nanocomposites were then cold pressed and sintered at 1400 0 C to obtain suitable condensed pieces. X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Particle Size Analyser (PSA) and Nanoindentation tester were used to characterize produced powders and sintered samples. According to Miedema model, intermetallic compound has the lowest Gi free energy compared with solid solution and amorphous phases and so is the first phase formed in the mechanical alloying of Ti 62.5 Si 37.5 powder mixture. Mechanical alloying results showed that nanostructured Ti 5 Si 3 and Ti 5 Si 3 -Al 2 O 3 nanocomposite were formed gradually during milling with final crystallite size of 15 and 13nm after 45h. TEM results also confirmed nanometric crystallite size of produced powders. In-situ fabrication of Al 2 O 3 reinforcements by reduction of TiO 2 with Al, caused fine distribution of nanometric Al 2 O 3 . The macrohardness of bulk Nanostructured Ti 5 Si 3 and Ti 5 Si 3 -15Wt.% Al 2 O 3 nanocomposite were evaluated as 950 and 1100 HV. Fracture toughness of these two compounds were calculated as 4 and 5.5 MPa.m 1/2 , respectively. Also elastic moduli of these two compounds were determined as 187 and 238 GPa, respectively. It is well known that fracture toughness of a material is not dependent only to composition but also ability of its microstructure to dissipate deformation energy without propagation of cracks. In brittle materials, such as ceramics and intermetallics,