Ring spun yam is the most widely used yarn type, especially for quality staple yarns. But the exact structure of the ring spun yarn is not known and the structure is explained only qualitatively. This study suggests a method for determining the structural characteristics quantitatively, defining a packing density (unction and a migration factor. The results show that ring spun yarn has a maximum packing density at around 2/3 of the yarn radius. The fibers in the yarn center are arranged parallel or with a small angle to the yarn axis, which is different from what has been known to us. The migration factor of a ring spun yarn ranges from 0.76 to 1.29. depending on the twist level that affects the fiber migration positively.