In this study, in situ (TiB+La2O3)/Ti-6Al-4V composites were processed by equal-channel angular pressing (ECAP) from 1 to 3 passes, and the microstructural evolution was quantitatively investigated. The results show that dislocation slip and recombination are the main formation mechanisms of the ultrafine-grained structure. Geometrically necessary boundaries (similar to 200nm) and incidental dislocation boundaries formed cell blocks. Deformation twins were observed in ECAPed titanium matrix composites (TMCs) after the second and third passes. The grain refinement after the first pass was the most remarkable but was not homogeneous. The average grain size (AGS) was further reduced to 0.28 nm after the second pass, and the microstructure became homogeneous with increasing equivalent strain. The AGS changed little, but the fraction of high-angle grain boundaries with angles above 72 degrees increased to approximately 19% after the third pass. TiB short fibers and La2O3 particles influenced the formation of ultrafine grains (UFGs) at the matrix/reinforcement interface region in different ways. Small La2O3 particles tend to reinforce TMCs by hindering dislocation motion through a Zener drag effect. By contrast, TiB short fibers facilitate recrystallization and the formation of UFGs through nucleation stimulated by particles and interaction with dislocations.