  We study the gauge coupling unification of the minimal supersymmetric
standard model with non-universal soft scalar and gaugino masses. The
unification scale of the gauge couplings is estimated for non-universal cases.
It is sensitive to the non-universality. It turns out that these cases can be
combined with the assumption of string unification, which leads to a prediction
of $\sin^2 \theta_W(M_Z)$ and $k_1$, the normalisation of the $U(1)_Y$
generator. String unification predicts $k_1=1.3 - 1.4$. These values have
non-trivial implications on string model building. Two-loop corrections are
also calculated. Some of these cases exhibit a large discrepancy between
experiment and string unification. We calculate string threshold corrections to
explain the discrepancy.
