Phase composition change and mechanical properties at room temperature of MoSi2 materials with the addition of aluminum were investigated. An explanation was given to the appearance of Mo5Si3 when hot pressing MoSi2 raw powder containing oxygen. The mechanical properties including Vickers hardness, bending strength and fracture toughness were improved with the addition of aluminum up to the limit content needed for absorbing the oxygen in MoSi2 raw powder. More aluminum addition than the limit content (in this study it is 5 wt %) will result in the formation of Mo(Si, Al)(2) and Si. The in situ formed Al2O3 could act as a crack pinning element. However, because the thermal expansion coefficients of Al2O3 and MoSi2 are near and there is a strong bonding between them, the toughening effect by such in situ formed small Al2O3 particles (less than 2 mu m) is limited.