  We suggest a simple supersymmetric SO(10) grand unified theory in 6
dimensions which produces the suitable fermion mass hierarchies. The 5th and
6th dimensional coordinates are compactified on a $T^2/Z_2$ orbifold. The gauge
and Higgs fields propagate in 6 dimensions while ordinal chiral matter fields
are localized in 4 dimensions. The orbifolding and boundary conditions realize
the gauge symmetry reduction,
  $SU(3)_C\times SU(2)_L \times U(1)_Y \times U(1)_X$, and the triplet-doublet
mass splitting. We introduce extra three sets of vector-like heavy fields: two
sets propagate in 6 dimensions and chiral fields which couple to them are
defined as the 1st generation, and one set propagates in 5 dimensions and
chiral fields which couple to them are defined as the 2nd generation. The
suitable fermion mass hierarchies are generated by integrating out these
vector-like heavy fields.
