Abstract: The hairpin vortex packets structure and characteristicscale in the later stage of bypass transition induced by cylinder wake areinvestigated by TR PIV technique for both the side-view and top-view cases.The two-dimensional velocity components are correlated with \lambda_ci criterion, and linear stochastic estimation is used to estimate theconditional average of the velocity field. For the side-view case, theconditionally averaged structure consists of a series of swirling motionslocated along a line inclined at a larger angle (17^\circ) from the wall, anda low-speed region occupied by cylinder wake appearing right above it in thefree stream. In the (x, z)-plane at the top of the log layer (y/δ=0.2), the dominant motions are shown to be large-scale regions of lowmomentum elongated along the streamwise and within a spanwise width of0.55δ, whichare consistently bordered by small-scale counter-rotating vortex pairsorganized in the streamwise. The results suggest that in the later part ofthe transition zone, due to the upward induction of wake vortex, the hairpinvortex packets statistically perform a skeleton frame of larger scale androbust physique.