Abstract: An experimental study was carried out to investigate the effect of local forcing on the friction drag of a turbulent boundary layer.The local forcing was given to the boundary layer flow by means of a sinusoidally oscillating jet issuing from a thin spanwise slot at the wall.The relationship among some inner-scales physical parameters,e.g.the mean-velocity profile,skin friction velocity and fluid dynamics viscosity,and the relationship between skin friction velocity and skin friction shear stress were taken advantage of accurately calculating skin friction drag.The velocity signals were decomposed into multi-scale eddy structures by wavelet transform.A conditional sampling technique for multi-scale coherent structures detection from simultaneous turbulent field was introduced using multi-scale instantaneous intensity factor of wavelet coefficients.Coherent structures for different scales were extracted by this technique and the effect of blowing and suction on friction drag was investigated in flat-plate turbulent boundary layer.The results reveal that friction drag is increased by the local forcing near the thin slot and decreased at downstream region.