Abstract: In the dilute surfactant drag-reducing flow, the slight viscoelasticity ofsolution affects the energy dissipation process of the flow, and this flowexhibits special fashion different from laminar flow or turbulent one inNewtonian fluid. The study on the modification of turbulent structures bythe viscoelasticity of the solution is important to clarify the dragreduction mechanism of surfactant solution flow. In this paper, dragreducing flow of dilute surfactant solution in the two dimensional rectanglechannel has been investigated experimentally. To grasp the instantaneousvelocity u-v in x-y plane, particle image velocimetry (PIV) is employed. Theexperiments pay attention to Reynolds number ranging from 10000 to40000 and concentration of additive CTAC (a kind of cationicsurfactants) from 25 to 100ppm on the flow.As for the additive CTAC concentration of 25, 40, 60 and 100ppm, the fanningfriction factor decreases and drag reduction (DR) increases with theincrease of mass concentration below the critical Reynolds number. Thesurfactant solution with 40ppm at Re=40000, compared with waterflow, shows DR of more than 70%.The instantaneous distribution of velocity, vorticity, and correlation ofvelocity fluctuation is analyzed. Statistics calculation is carried outbased on 500 velocity fields which are measured by PIV. The results exhibitthat the drag-reducing flow shows the approximate characteristics of thelaminar flow. The wall-normal velocity fluctuation of drag-reducing flow isdamped, which causes the decrease of turbulence transportation and Reynoldsshear stress comparing with the turbulent water flow. The streamwisevelocity fluctuation of drag-reducing flow shows the streamwise developedband-like distribution. This pattern indicates the new characteristic ofturbulence transportation in drag-reducing flow.