Fiber migration of ring spun yarns of different twist levels produced under different spinning tension was measured using the image processing technology. The experimental results reveal several new structural characteristics which can be affected by twist. The decrease of the mean fiber position as the twist multiplier (T.M.) increases shows the fact that the yarn packing density near the yarn axis becomes greater than that in the peripheral zone as more twists is introduced into the yarn. The reduction of the rms deviation with increase of the T.M. values means that the individual fibers cannot be allocated in a wide range of the yarn cross-section as the number of twists increases. In other words, twists constrain the fiber motion in such a way that the magnitude of the radial position variation of fibers along the yarn axis decreases. The constituent fibers, however, alter the radial positions in a more intensive extent as more twists are given to the yarn. from the results, it is also found that one migration occurs as 4~5 turns of twist are inserted to the yarn. The orientation angle increases as the number of twist in the yarn increases, which supports the results for the effect of twist on migration intensity. And the most frequent orientation angles are obtained at the radial position 0.5 of the yarn cross-section independent of the yarn twists. But the spinning tension does not affect fiber migration. The twist affects the yarn hairiness through fiber migration.