  We discuss the role of break-up process of a loosely-bound projectile in
subbarrier fusion reactions. Coupled-channels calculations are carried out for
$^{11}$Be + $^{208}$Pb and $^{4,6}$He + $^{238}$U reactions by discretizing in
energy the particle continuum states. Our calculations show that the coupling
to the break-up channel has two effects, namely the loss of flux and the
dynamical modulation of fusion potential. Their net effects differ depending on
the energy region. At energies above the Coulomb barrier, the former effect
dominates over the latter and cross sections for complete fusion are hindered
compared with the no coupling case. On the other hand, at below the barrier,
the latter effect is much larger than the former and complete fusion cross
sections are enhanced consequently.
