  We present a model-independent calculation of hadron matrix elements for all
dimension-six operators associated with baryon number violating processes using
lattice QCD. The calculation is performed with the Wilson quark action in the
quenched approximation at $\beta=6/g^2=6.0$ on a $28^2\times 48\times 80$
lattice. Our results cover all the matrix elements required to estimate the
partial lifetimes of (proton,neutron)$\to$($\pi,K,\eta$) +(${\bar
\nu},e^+,\mu^+$) decay modes. We point out the necessity of disentangling two
form factors that contribute to the matrix element; previous calculations did
not make the separation, which led to an underestimate of the physical matrix
elements. With a correct separation, we find that the matrix elements have
values 3-5 times larger than the smallest estimates employed in
phenomenological analyses of the nucleon decays, which could give strong
constraints on several GUT models. We also find that the values of the matrix
elements are comparable with the tree-level predictions of chiral lagrangian.
