The hydrolysis properties of peptide nanospheres consisting of a high-density poly(ethylene glycol) (PEG) brush layer and a poly(L-phenylalanine) (PPhe) core were evaluated in relation to their self-assembled nanostructure. To clarify the effect of the self-assembled structure of the nanospheres on their hydrolysis properties, deformed nanospheres possessing exactly the same components were used as a comparison sample. The desorption of the PEG chains on the peptide nanospheres occurred weakly in alkaline solution at 80° C for 7 days, whereas the deformed nanospheres showed drastic desorption of 70% of the PEG chains within 3 days, possibly due to a low density of PEG chains on their surfaces. The content ratio of the β-sheet conformation in the core increased with increasing hydrolysis time, suggesting that the α-helix and random coil conformations were more easily hydrolyzed. Furthermore, the peptide nanospheres retained all their properties, even after autoclaving and ethanol sterilization. These results demonstrated that the self-assembled spherical morphology induced extreme stability and sterilizable peptide nanospheres can be useful as drug-delivery system.