Quark-hadron duality is a basic assumption which many perturbative QCD calculations rely on. There are plausible arguments about why it is
valid, but only at a qualitative level. We propose a set of quantities for which quantitative and non-perturbative calculation is possible using lattice QCD. A well-known example is related to the R-ratio of e+e - scattering. A primary target of the method discussed in this talk is the inclusive decays of B mesons. We discuss on how one can calculate its decay rate or some related quantities.