The vortex core detection method based on the Liutex vector is utilized to investigate the alternation of vortical structures on a boundary layer transition subjected to spanwise-wall oscillation. Compared with iso-surface based methods, the Liutex core line method is shown to be precise, free of threshold and capable to capture both strong and weak vortices simultaneously. Tollmien-Schlichting (T-S) waves in the linear growth region,Λ- and hairpin vortices in the transition region and twisted vortices in the turbulent region are all well captured by Liutex core lines. The cyclic wall movement accelerates the transition process while reducing the turbulent drag by 21.8％ with selected parameters. For the wall oscillation case, the development from T-S wave toΛ-vortex is advanced about one T-S wave length in the streamwise direction. In the transition region, theΛ-vortex and legs of hairpin vortex are shortened in the wall oscillation case, and the symmetry of the vortical structures is lost in the late transition region since the introduction of asymmetry disturbances by the cyclic wall movement. Extrusions of weak vortices at the edge of boundary layer are found in the turbulent section which is often omitted by iso-surface based vortex identification method. Thus, it is demonstrated that for the transitional boundary layer the Liutex core line method provides a systematic and threshold-free vortex definition, which could serve as a powerful tool to understand and guide flow control.