Abstract: At late stages of compressible transitional boundary layers over a flat-plate, ejection and sweep flows are investigated by means of direct numerical simulation with high order accuracy. Numerical results with high resolution clearly represent complex phenomena of ejection and sweep flows in the transition process of boundary layers. Close relationships with typical vortex structures and spike structures are revealed. Details of formation mechanisms of ejection and sweep flows are analyzed. Numerical results demonstrate experimental findings. Corresponding mechanisms are analyzed.