  The B-meson decay constant fB has been calculated from unquenched lattice QCD
in the unphysical region. For extrapolating the lattice data to the physical
region, we propose a phenomenological functional form based on the effective
chiral perturbation theory for heavy mesons, which respects both the heavy
quark symmetry and the chiral symmetry, and the non-relativistic constituent
quark model which is valid at large pion masses. The inclusion of pion loop
corrections leads to nonanalytic contributions to fB when the pion mass is
small. The finite-range regularization technique is employed for the
resummation of higher order terms of the chiral expansion. We also take into
account the finite volume effects in lattice simulations. The dependence on the
parameters and other uncertainties in our model are discussed.
