The polarizabilities of linear molecular aggregates are calculated by using the numerical coupled Lioville approach (NCLA), which can describe the intermolecular interaction by using the propagation of retarded electric fields among the monomers. The polarizabilities for intermediate-size molecular aggregates under near-resonant intense external fields are found to exhibit abrupt changes such as the phase-transition behaviors as the field intensity increases. The polarizability at each molecule site is investigated by elucidating the variation in the amplitude and the phase of a polarization at each molecule site.