  We consider $b$-$\tau$ unification in supersymmetric $SU(5)$ grand unified
theories (GUTs) with extra matters. The renormalization group runnings of $b$
and $\tau$ Yukawa coupling constants may be significantly affected by the
existence of extra matters. If the extra matters interact with the standard
model particles (and their superpartners) only through gauge interaction, the
ratio of the $b$ to $\tau$ Yukawa coupling constants at the GUT scale becomes
suppressed compared to the case without extra matters. This is mainly due to
the change of the renormalization group running of the $SU(3)_C$ gauge coupling
constant. If the extra matters have Yukawa couplings, on the contrary, the
(effective) $b$ Yukawa coupling at the GUT scale can be enhanced due to the new
Yukawa interaction. Such an effect may improve the $b$-$\tau$ unification in
supersymmetric GUTs.
