  Generalizing the kink operator of the Heisenberg spin 1/2 model, we construct
a set of Klein factors explicitly such that $(1+1)$ dimensional fermion
theories with arbitrary number of species are mapped onto the corresponding
boson theories with the same number of species and vice versa. The actions for
the resultant theories do not possess any nontrivial Klein factor. With this
set of Klein factors, we are also able to map the simple boundary states such
as the Neumann and the Dirichlet boundary states, of the fermion (boson) theory
onto those of the boson (fermion) theory. Applications of the Fermi-Bose
equivalence with the constructed Klein factors to well-known $(1+1)$
dimensional theories have been discussed.
