Short wool fibers obtained by the stretch breaking process can be blended with cotton fibers and processed in a cotton spinning system, which has a high production rate. For the structural property of the wool fiber after stretch breaking, the diameter and length of the wool fiber were measured as a function of time. The diameter of the broken fibers was finer than the diameter of untreated fibers. The fiber diameter at the break point was the finest and was more irregular than the original fiber. The broken fiber showed mechanical properties of increased modulus, decreased breaking strain, and increased breaking strength.