  Because quarks in hadronic states are subjected to strong QCD forces, it is
not straight-forward to apply calculation obtained at the quark level to
physical processes involving hadrons. In this paper we present a new approach
to treat the semileptonic decays of pseudoscalar mesons. At the quark level,
the process is straightforward. However, when we fold the quark-level process
to the meson decay process, the complication from QCD effects come in. We make
the fundamental assumption that the vertex of type M-q-qbar can be approximated
by an effective constant gamma-5 coupling. With this assumption, the hadronic
process can then be related to the quark-level process. The model is applied to
the semileptonic decays of pi, K, D and B mesons.
