Coarse and fine TiB whiskers were formed during the reaction synthesized TiB reinforced Ti matrix composites. The microstructure of the as-synthesized TiB whiskers has been investigated by TEM and HREM. Dislocations and stacking faults were observed in the coarse TiB whiskers, and only high-density stacking faults presented in the fine TiB whiskers. The formation of dislocations is related to the stacking faults, the formation mechanism and atomic structure of dislocations in the in situ synthesized TiB whiskers was suggested. (C) 2009 Elsevier B.V. All rights reserved.