In order to study the mechanisms of the turbulent frictional drag reduction by surfactant additives,the drag reduction,the shear viscosity and the shear stress relaxation were measured for solutions of a cationic surfactant cetyltrimethyl ammonium bromide (CTAB) with the same molar sodium salicylate as a counter-ion.It is found that the first step relaxation time decreases with increasing concentration and,thus,with the maximum drag-reducing rates,which indicates that the stiffness of the micellar structures with the first relaxation time,increases with the increase of the concentration of CTAB.Furthermore,for this surfactant,a viscoelastic property is necessary for reducing drag,while a stronger viscoelasticity characterized by a tail relaxation time does not necessartly mean a higher drag-reducing rate.