  Dynamical quasi-localization of matter fields in extra dimension is an
elegant mechanism to generate hierarchical 4-dimensional Yukawa couplings. We
point out that a bulk matter field whose zero mode is quasi-localized can give
a large Kaluza-Klein threshold correction to low energy gauge couplings, which
is generically of the order of ln (Yukawa)/8pi^2, so it can significantly
affect gauge coupling unification. We compute such threshold corrections in
generic 5-dimensional theories compactified on S^1/Z_2 X Z_2, and apply the
result to grand unified theories on orbifold generating small Yukawa couplings
through quasi-localization.
