  We study a supersymmetric SU(5) model with the extra Higgs multiplets of
45+\bar 45. The unification of the gauge couplings, the fermion masses and the
proton lifetime are discussed in details. The dimension-five operators mediated
by different colored Higgs sector can be destructive with each other. This
effect serves a way of solving the longevity of proton. We analytically analyze
this destructive effect in a special limit where the mixing between the 5- and
45-plets is small. Although the theory does not hold in this special limit, it
is a revelatory starting point. We can relax this limit and retain the
destructive effect. In a generalized parameter space, this model is in accord
with experimental results.
