In situ TiB whiskers reinforced Ti metal matrix composites have been fabricated by spark plasma sintering (SPS). Microstructure and stacking faults of in situ TiB whiskers in the composites were investigated in detailed by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM). The TiB whiskers exhibit a hexagonal shape at the transverse section and grow along the TiB direction. The crystallographic planes of the TiB whiskers at transverse section are always of the (100), (101) and (10 1 ) planes. The interfaces between the in situ TiB and Ti matrix are clean and faceted along (100)TiB side. However, small lateral growth steps are observed along (101) and (10 1 ) planes of TiB. The stacking faults in TiB whiskers are typically with a stacking fault plane of (100)TiB and displacement vectors of 0.5a±0.254c. The formation mechanism of stacking faults has been proposed and verified by HREM observations.