Abstract: Riblets perform a drag-reducing effect on the turbulent boundary layers (TBLs) in a certain range of their peak-to-peak spacings, s⁺. However, the drag reduction produced by the riblets breaks down after s⁺ increasing beyond its optimum value, and eventually the drag reduction becomes an increase case. García-Mayoral and Jiménez (2011) believed that the breakdown is due to the onset of a Kelvin-Helmholtz instability in the TBL over riblets, which introduces a kind of Kelvin-Helmholtz-like roller structures. In the present study, these spanwise rollers, for the first time in experiments, were clearly observed by the smoke flow visualization technique. And their conceptual model was outlined lastly as well.