Isomerization and peptide bond cleavage at aspartic residue (Asp) in peptide models have been reported. In this study, the mechanisms and energies concerning the isomerization and peptide bond cleavage at Asp residue were investigated by the density functional theory (DFT) at B3LYP/6-311++G(d,p). The integral equation formalism-polarizable continuum model (IEF-PCM) was utilized to calculate solvation effect by single-point calculation of the gas-phase B3LYP/6-311++G(d,p)-optimized structure. Mechanisms and energies of the dehydration in isomerization reaction of Asp residue were comparatively analyzed with the deamidation reaction of Asn residue. The results show that the succinimide intermediate was formed preferentially through the step-wise reaction via the tetrahedral intermediate. The cleavage at C-terminus is more preferential than those at N-terminus. In comparison to isomerization, peptide bond cleavage is ≈ 20 kcal mol(-1) and lower in activation barrier than the isomerization. So, in this case, the isomerization of Asp is inhibited by the peptide bond cleavage.