The tensile properties and microstructures of AA6061/SiCp composites fabricated by the pressureless infiltration method under a nitrogen atmosphere were examined. Since the spontaneous infiltration of molten AA6061 into the powder bed containing SiCp occurred at 800 degreesC for 1 hour under a nitrogen atmosphere, it was possible to fabricate composites reinforced with SiCp. Reaction product (Al4C3) was formed at the interface between SiCp and Al alloy matrix. In addition, the amount and size of the Al4C3 is increased significantly by increasing the infiltration temperature. The reaction product (AlN) was formed as a result of the in situ reaction in both the control alloy and the composite. A significant strengthening even in the control alloy occurred due to the formation of in situ AlN particle even without an addition of SiCp. While a further strengthening of the composite was produced by the reinforced SiCp, strain to failure of the composite fabricated at 800 degreesC showed the lowest value (1.3%) in the T6 condition due to the formation of the severe reaction product (Al4C3). The grain size of the control alloy significantly decreased to about 20 mum compared to 50 mum for the commercial alloy. In addition, the grain size in the composite reinforced with SiCp further decreased to about 8.0 mum. This grain refinement contributed to strengthening of the control alloy and composite.