The critical plastic shear strain corresponding to the onset of adiabatic shear band for Ti-6Al-4V alloy is calculated based on the curve-fitting least-squares method. Using gradient-dependent plasticity, the local plastic shear deformation distribution in the adiabatic shear band is obtained theoretically to fit the experimental flow lines by Liao and Duffy (1998). The critical plastic shear strains at different values of the adiabatic shear band width are assessed. When the width is assigned to be in the range of 1-2 mm, the predicted critical plastic shear strain is in the range of 0.1-0.47. It is found that the theoretical expression for the local plastic shear deformation distribution in the adiabatic shear band can well describe the nonlinear characteristics of flow lines. When the width is assigned to be a higher value, the theoretical flow lines in the adiabatic shear band are more curved at the two ends of the band, while when the width is assigned to be a lower value, the theoretical flow lines are straighter and only slightly curved.