In situ Al3Ti/Al composites were fabricated by a combination of vacuum hot pressing (VHP) and friction stir processing (FSP). The formation mechanism of the Al3Ti and the effect of VHP and FSP parameters on the resultant microstructure and mechanical properties were investigated. The Al3Ti formed due to the reactive diffusion between Al and Ti during VHP, and the number of Al3Ti particles increased with increasing the temperature and holding time of the VHP. FSP not only induced the Al-Ti reaction, but also resulted in significant refining of the Al3Ti, thereby creating a homogeneous distribution of Al3Ti particles in the Al matrix. These microstructural changes led to significant improvement in the tensile properties of the in situ Al3Ti/Al composite. However, the change trends of the tensile properties of the FSP samples were dependent on the extent of the Al-Ti reaction during VHP. (C) 2011 Elsevier B.V. All rights reserved.