  We study possibilities for lowering the decay constants of superstring
axions. In the heterotic Calabi-Yau compactification, a localized
model-dependent axion can appear at a nearly collapsing 2-cycle. The effect of
flux can be used for generating warp factor suppression of the axion decay
constant. We also point out that the hidden sector instanton potential much
higher than the QCD instanton potential picks up the larger effective axion
decay constant as that of the QCD axion. We show that this can be converted by
introducing many hidden-sector quarks so that the decay constant of the QCD
axion turns out to be much smaller than the string scale.
