Doughnut-shaped nanoreactors, peptide nano-doughnuts, were self-assembled from peptides and organic Au salts. We demonstrated that monodisperse Au nanocrystals were synthesized inside the cavities of peptide nano-doughnuts by the reduction of Au ions and the size of the Au nanocrystal was controlled by the cavity dimension. The Au nanocrystals inside the nano-doughnuts were extracted by destroying the nano-doughnuts via long UV irradiation (>10 h). These features may allow the peptide nano-doughnuts to be applied in the fields of nanomaterial syntheses, controlled release systems, and drug delivery.