A natural pentacyclic triterpenoid oleanolic acid 1 and its biotransformed metabolites 2-3 are potential α-glucosidase inhibitors. To elucidate the inhibitory mechanism of compounds 1, 2 and 3 against α-glucosidase, we calculated (i) their electronic and optical properties using DFT and TD-DFT at the B3LYP/6-31G(d) level in gas and IEF-PCM solvent; and (ii) their binding energies to α-glucosidase via docking study. DFT results showed that the α-glucosidase inhibtion is mainly depend on the polarity parameters of the studied compounds. Docking results revealed that the activity increased with binding energies (i.e. the stability of ligand-receptor complex). The specroscopic data of oleanolic acid 1 and its metabolites 2 and 3 are well predicetd for 13C NMR chemical shifts (R2=99%) and 1H NMR chemical shifts (R2=90%); and for (ii) UV/vis spectra. The assignments and interpretation of NMR chemical shifts and bathochromic shift of λMAX absorption bands are discussed.