The single-crystal yield surfaces of bcc crystals for slip on {123}<111> systems have been analyzed systematically and derived based on the Taylor/Bishop-Hill theory. It is found that there are 338 stress states altogether. All the stress states are classified according to the crystal symmetry. The results demonstrated that there are 14 groups of yield vertices, which activate five, six, or eight slip systems, depending on the crystallographically nonequivalent groups. In addition, the four groups of yield vertices that Chin and Wonsiewicz[15] did not consider have been obtained(there are 126 stress states). Among all these vertices, the fraction of vertices for which there is slip ambiguity (more than five active slip systems) is reduced considerably to 146/338 (≈43 pct). By means of the stress states obtained by Chin et al. and by the present authors, respectively, the plastic flow for axially symmetric deformation and three-axial deformation have been analyzed. In the case of axially symmetric deforma-tion, the results are the results are the same using the two different kinds of stress states. The obtained new stress states are necessary in order to analyze the three-axial deformation.