In situ TiB whisker reinforced Ti-4.0Fe-7.3Mo matrix composites have been fabricated by spark plasma sintering (SPS). The microstructure of the in situ TiB whiskers and interfacial structures in the composites were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution electron microscopy (HREM). The interfaces between in situ TiB and the Ti matrix are clean and faceted along the (100)TiB plane. However, small lateral growth steps are observed along the (101) and (101h) planes of TiB. Because of the characteristics of the TiB crystal structure, the growing velocity of the (100)TiB plane along the [010]TiB direction is much faster than that of the [001]TiB direction, which leads to TiB growth as a whisker shape along the [010]TiB direction with a hexagonal transverse section. The growth mechanism of TiB whiskers is a stacking process of the (100)TiB plane.