  We study classical and quantum aspects of D=4, N=2 BPS black holes for T_2
compactification of D=6, N=1 heterotic string vacua. We extend dynamical
relaxation phenomena of moduli fields to background consisting of a BPS soliton
or a black hole and provide a simpler but more general derivation of the
Ferrara-Kallosh's extremized black hole mass and entropy. We study quantum
effects to the BPS black hole mass spectra and to their dynamical relaxation.
We show that, despite non-renormalizability of string effective supergravity,
quantum effect modifies BPS mass spectra only through coupling constant and
moduli field renormalizations. Based on target-space duality, we establish a
perturbative non-renormalization theorem and obatin exact BPS black hole mass
and entropy in terms of renormalized string loop-counting parameter and
renormalized moduli fields. We show that similar conclusion holds, in the large
T_2 limit, for leading non- perturbative correction. We finally discuss
implications to type-I and type-IIA Calabi -Yau black holes.
