The structures of C- and N-terminally monoPEGylated human parathyroid hormone fragment hPTH(1-34) as well as their unmodified counterparts, poly(ethylene glycol) (PEG) and hPTH(1-34), have been studied by small-angle neutron scattering (SANS). The scattering results show that free hPTH(1-34) in 100 mM phosphate buffer (pH 7.4) aggregates into clusters. After conjugation with PEG, the PEG-peptide conjugates self-assemble into a supramolecular core-shell structure with a cylindrical shape. The PEG chains form a shell around the hPTH(1-34) core to shield hPTH(1-34) from the solvent. The detailed structural information on the self-assembled structures is extracted from SANS using a model of the cylindrical core with a shell of Gaussian chains attached to the core surface. On the basis of the data, because of the charge-dipole interactions between the conjugated PEG chain and the peptide, the conjugated PEG chain forms a more collapsed conformation compared to free PEG. Moreover, the size of the self-assembled structures formed by the C-terminally monoPEGylated hPTH(1-34) is about 3 times larger than that of the N-terminally monoPEGylated hPTH(1-34). The different aggregation numbers of the self-assembled structures, triggered by different PEGylation sites, are reported. These size discrepancies because of different PEGylation sites could potentially affect the pharmacokinetics of the hPTH(1-34) drug.