Using the measured experimental data for 6,7,8 Li+208 Pb system, we investigated the relation between the interaction distance and the lowest threshold separation energy of the projectiles. Although 6 Li can be considered more weakly bound projectile than 8 Li, the lithium isotope with the largest interaction distance is not 6 Li but 8 Li. Also, we can infer that the difference of the semiexperimental direct reaction (DR) cross sections and the total breakup / transfer reaction cross sections comes from α and 7 Li capture. Finally, the barrier height, VB , of 8 Li + 208 Pb system compared with 7 Li + 208 Pb system are reduced about 3MeV. This is the rise of flux from exp the elastic into the reaction channel, which tends to enhance σ R exp . As a result, we can deduce that the valence neutron play a role as the lowering of the barrier height.