  We present preliminary results of a new lattice computation of hadronic
matrix elements of baryon number violating operators which appear in the
low-energy effective Lagrangian of (SUSY-)Grand Unified Theories. The
contribution of irrelevant form factor which has caused an underestimate of the
matrix elements in previous studies is subtracted in this calculation. Our
results are 2$\sim$4 times larger than the most conservative values often
employed in phenomenological analyses of nucleon decay with specific GUT
models.
