To understand the differences in the mechanical properties between the center and surface regions of large size forged TC18 titanium alloy bar, electron backscatter diffraction (EBSD) technique is applied to reveal the differences in textures and strains in β phase in addition to the microstructures observation. The influence of the states of β phase on the deviation to the Burgers orientation relationship (OR) between α/β phases is also analyzed according to the acquired EBSD information. It is found that the β phases in different positions of forged bar are in different states of strain, textures and grain sizes. The β phase in the center shows strong <100> texture and coarse and inhomogeneous grain sizes which all contribute to the difference in properties. The spheroidization of α_p phase proceeded mainly within β grains at subgrain boundaries and the OR between two phases changed slightly as the lamella α_p phase transforms into block-like α_p phase, but changed strongly during transforming to globular morphology. The misorientation in β phase is an effective parameter to evaluate the contribution of work-hardening state for the enhancement of strength, whereas the level of the deviation to Burgers OR is an effective parameter to evaluate the spheroidizing rate and the recovery extent.