The tension of a towing rope which is partly immersed in, or floating on the water is the sum of the resistance of the towed body and the water resistance of the rope itself. When the towed body is small, the resistance of the towed body is comparable with, or often smaller than, the water resistance of the rope which is the function of the immersed length, diameter and surface roughness of the rope and the towing velocity. A theoretical investigation into the form of a towing rope is worked out and the estimating method of the rope tension is presented. The water resistances of several plastic ropes are measured in experiment tanks and the coefficient of resistance c_f _??_0.008 is obtained.