Titanium matrix composites reinforced with TiB, TiC and Y2O3 are fabricated by a non-consumable arc-melting technology utilizing chemical reaction between titanium, B2O3, B4C and Y. Microstructure of in situ synthesized Y2O3 was observed by scanning electron microscope (SEM), transmission electron microscope (TEM) and high-resolution transmission electron microscope (HREM). The Y2O3 forms in a way of nucleation and growth. Primary Y2O3 tends to grow in dendritic shape and becomes coarse. Secondary Y2O3 mostly grows in sphericity and the size is very small. Moreover, there is no fault in the Y2O3 reinforcements. The interface between Y2O3 and titanium was observed by means of TEM and HREM. Three groups of crystallographic relationships between Y2O3 and Ti were found. The interface between the Y2O3 and the titanium matrix is very clean and well bonded, and there is no interfacial reaction.