  We suggest a simple grand unified theory where the fifth dimensional
coordinate is compactified on an $S_1/(Z_2 \times Z_2')$ orbifold. This model
is based on the supersymmetric flipped $SU(5) \times U(1)$ grand unified
theory, which can realize not only the triplet-doublet splitting but also the
natural fermion mass hierarchies. The triplet-doublet splitting is realized by
$S_1/(Z_2 \times Z_2')$ orbifolding, which also reduces the gauge group as
$SU(5) \times U(1) \to SU(3)_c \times SU(2)_L \times U(1)_Z \times U(1)_X$. The
suitable fermion mass hierarchies are generated by integrating out extra three
sets of vector-like heavy fields which can propagate in five dimensions. The
radiative corrections to the large Yukawa coupling of right-handed neutrinos
can reduce the gauge group to $SU(3)_c \times SU(2)_L \times U(1)_Y$.
