Abstract: This paper presents an experimental investigation of the spatial-temporal evolution of Lagrangian structures in the transition process of a boundary layer with a unit Reynolds number of 9.7×10⁶/m and Mach number of 6. The development of unstable waves and the associated wall temperature distribution are measured by an infrared camera; the turbulence generation process is observed by utilizing state-of-the-art ultrafast flow visualization methods and particle image velocimetry. The results show that the second mode waves are modulated by long-wavelength and low-frequency waves, forming wave packets at the late transition stage. The evolution of Lagrangian material lines, based on the velocity fields derived from particle image velocimetry, demonstrates that the wave packets undergo three steps. Firstly, near-wall fluids are lifted upward to be soliton-like waves; secondly, the fluids away from the wall are swept down over the bulge, yielding a localized strong shear layer; finally, the shear layer eventually evolves into vortical structures.