  We study non-universal corrections to the gauge couplings due to higher
dimensional operators in supersymmetric SU(5) grand unified theories. The
corrections are, in general, parametrized by three components originating from
{\bf 24}, {\bf 75} and {\bf 200} representations. We consider the prediction of
$\alpha_3(M_Z)$ along each {\bf 24}, {\bf 75} and {\bf 200} direction, and
their linear combinations. The magnitude of GUT scale and its effects on proton
decay are discussed. Non-SUSY case is also examined.
