  A 5D black hole(M$_5$) is investigated in the type IIB superstring theory
compactified on S$^1 \times $T$^4$. This corresponds to AdS$_3 \times $S$^3
\times $T$^4$ in the near horizon with asymptotically flat space. Here the
harmonic gauge is introduced to decouple the mixing between the dilaton and
others. On the other hand we obtain the BTZ balck
hole(AdS$_3\times$S$^3\times$T$^4$) as the non-dilatonic solution. We calculate
the greybody factor of the dilaton as a test scalar both for a 5D black
hole(M$_5 \times $S$^1 \times $T$^4$) and the BTZ black hole(AdS$_3 \times
$S$^3 \times $T$^4$). The result of the BTZ black hole agrees with the greybody
factor of the dilaton in the dilute gas approximation of a 5D black hole.
