  The greybody factors in BTZ black holes are evaluated from 2D CFT in the
spirit of AdS$_3$/CFT correspondence. The initial state of black holes in the
usual calculation of greybody factors by effective CFT is described as
Poincar\'{e} vacuum state in 2D CFT. The normalization factor which cannot be
fixed in the effective CFT without appealing to string theory is shown to be
determined by the normalized bulk-to-boundary Green function. The relation
among the greybody factors in different dimensional black holes is exhibited.
Two kinds of $(h,{\bar h})=(1,1)$ operators which couple with the boundary
value of massless scalar field are discussed.
