Aluminum-based composites have many advantages over their conventional counterparts. A major problem in such composites is the clustering of particles in the matrix. Friction stir processing (FSP) can homogenize particle distribution in aluminum-based composites. In this study, unannealed TiO2 particles were used to prepare Al-TiO2 nanocomposite using FSP. The TiO2 particles, about 1 A mu m, were dispersed into an aluminum matrix by 6 passes of FSP. The TiO2 particles were fractured by multiple FSP passes, leading to a nano-size particle distribution in the matrix. Nanoscale dispersion was confirmed by scanning electron microscopy and transmission electron microscopy. The fractured TiO2 particles reacted with the aluminum matrix to form Al3Ti intermetallic and Al2O3 ceramic. The progression of the Al-TiO2 reaction from the fourth to the sixth pass of FSP was revealed by x-ray diffraction. Due to the nanoscale dispersion, the yield and ultimate tensile strength of the composite increased to 97 and 145 MPa, respectively. Ductility of the composite decreased marginally compared to the as-received aluminum. As the dispersed particles pin dislocations, the strain-hardening rate of the composite was considerably increased and the same was seen in the Kocks-Mecking plot. The TiO2 particles are mechanically activated due to their fracture during FSP, hence leading to reaction with the matrix. The particle refinement and dispersion lead to a homogeneous matrix with higher strength.