PURPOSE: The titled yarns, having a minimum radius of curvature within a specific range at the curve formed by the axis of a single yarn and plural crimps and capable of improving the humor treating efficiency.

CONSTITUTION: Permselective hollow yarns having plural crimps, preferably 1W500% amplitude based on the outside diameter of the single yarn and wavelength of 5W1,000 times based on the outside diameter of the single yarn. The minimum radius of curvature at the curve formed by the axis of the single yarn is within 2W100mm, preferably 10W50mm range, and coagulation of blood or residual blood is hardly caused in blood treatment. Preferably, the outside diameter of the single yarn is 100W400μ, and the membrane thickness is 5W50μ. For blood dialysis, the water permeability performance (UFR) is preferably within 1.0W30.0ml/(m2.hr.mmHg) range.