The phase development mechanism during drawing from a highly entangled melt of ultra-high-molecular-weight polyethylene is analyzed by simultaneous measurements of in situ X-ray diffraction using synchrotron radiation and stress/strain behavior. The stress/strain curve exhibits a plateau region at the initial stage of the draw, and no crystalline reflections appear on a series of in situ X-ray diffraction patterns. However, as the sample draw proceeds above a critical strain, a metastable hexagonal reflection appears and becomes predominant, where the stress/strain curve still shows a plateau deformation. With a further increase of the strain, the intensity of the hexagonal reflection peak begins to decrease and subsequently that of the usual orthorhombic ones increase. Correspondingly, a rapid increase of draw stress, because of the strain-hardening behavior, is recorded.