  In this paper, we appliy the orbifold GUT mechanism to the SU(5) model in
noncommutative geometry, including the fermonic sector. Imposing proper parity
assignments for "constituent fields" of bosons and fermions, the couplings
between fermions and the heavy bosons $X_{\mu}, Y_{\mu}$, and $ H^{c}$ are
prohibited by the parity symmetry. As a result, the derived fermionic
Lagrangian is just that of the standard model, and proton decay is forbidden at
tree level. If quantum fluctuation respects the parity symmetry, the process
will be naturally suppressed or even forbidden completely.
