Compared with equiaxed microstructure, the / titanium alloys with lamellar microstructure show higher fracture toughness but lower strength and ductility. Crack tip plastic zone (CTPZ) size is an important factor that influences the fracture toughness. In the present study, the CTPZ of Ti-6Al-4V with lamellar and equiaxed microstructures was characterized and compared to reveal the influence of CTPZ on fracture toughness. For lamellar microstructure, large-scale slip, coordinated deformation at grain boundaries and deformation twinning will all expand the range of the plastic zone, inducing the measured range of CTPZ to be obviously larger than that in equiaxed microstructure. Both the expanded CTPZ and the enriched plastic deformation of lamellar microstructure could promote the capacity of absorbing energy and thus increase the fracture toughness.