The viscoelastic segment has been estimated from the limiting modulus at high frequencies with the aid of the modified stress-optical rule. For the case of polymer melts, experimental studies show that thus estimated viscoelastic segment size is close to the Kuhn segment size, which is calculated from the chain dimension. Molecular origin of this correspondence is revisited with the theoretical consideration of the initial chain orientation after step strains. By assuming pseudo-affine orientation of repeating units, we show that the correspondence between the viscoelastic and Kuhn segments can be naturally obtained in the frame of the stress-optical rule. Effects of the nonaffine orientation and the nematic interaction on the correspondence are also discussed.