In order to predict the nominal stress-strain curve of an isotropic polycrystal Ti-15V-3Al-3Cr-3Sn (hereafter, Ti-15-3(ST)) sheet in tension test, three-dimensional finite element (FE) analysis is elucidated considering three boundary conditions. According to the constraint at both ends of specimen, three boundary conditions are a simulated case based on the empirical data, a full case and a free case. In the simulated case, the nominal stress-strain curve can be well predicted until the fracture strain. Maximum load point does not mean the termination of uniform deformation in the present study. The onset of localized necking is intensively discussed and the origin of localized necking is concluded to be not due to the plastic instability but due to the deformation constraint at both ends of specimen.