Structural transformation from the herringbone structure to the preferentially oriented lamellae under the elongational flow has been studied by the two-dimensional small-angle X-ray scattering (2d-SAXS) technique. For this purpose, thermoplastic elastomer SBS (polystyrene-block-polybutadiene-block-polystyrene) triblock copolymer samples were used. The lamellar samples of which polybutadiene chains were chemically cross-linked in advance are subjected to be fractured by a uniaxial stretching at room temperature. Thus, the herringbone structure resulted. Then, the samples were further subjected to the elongational flow at an elevated temperature above the glass transition temperature of polystyrene. The process of the structural transformation was followed by the time-resolved 2d-SAXS measurement using synchrotron radiation upon temperature jump (T-jump) from room temperature for the uniaxially stretched SBS sample. A characteristic parallel streak pattern of 2d-SAXS was observed for the herringbone structure, which transformed rapidly into an oblique streak pattern upon the T-jump, gradually emerging into a two-spot pattern as time went on. Various structural parameters can be evaluated to characterize truncated lamellae and grains (size and orientation). On the basis of the results characterizing the transient structures, the mechanism of the structural transformation was considered. It was found that the process is different depending on the degree of cross-linking. However, commonly the process was found to be divided into three regions (I, II, and III). These regions are governed by the balance between perpendicular orientation of the truncated lamellae within the grain and parallel orientation of the PS lamellae with respect to the stretching direction. The corresponding orientational relaxation of the PB chains was also examined by the time-resolved Fourier transformed infrared spectroscopic measurements. The conformational. changes of the PB chain were found to play a role in controlling the lamellar repeat distance in regions I and II, to which respectively further chain stretching and orientational relaxation of the PB chain were relevant.