The mechanism and selectivity of the asymmetric cycloaddition reactions of 3-vinylindole towards methyleneindolinone (Diels-Alder reaction) or ketimine (aza-Diels-Alder reaction) were investigated theoretically by DFT and ONIOM methods. The non-catalytic DA reaction occurred in a two-stage one-step mechanism with the energy barriers of 17.0-29.6 kcal mol(-1). Reactivity index analysis indicated that the cycloaddition tended to occur by the most nucleophilic terminal C-4 center in 3-vinylindole and the most electrophilic C-beta center in methyleneindolinone with lower activation energies, which accounted for the regioseletivity observed in experiment. For non-catalytic reactions, the slightly exo-preference stemmed from the stabilizing p-p stacking of two indole rings in substrates. In the presence of chiral N, N'-dioxide-Ni(II) complex, the bulky ortho-iPr group in aniline of ligand provided sufficient steric shielding around dienophile from the si-face attack in Diels-Alder reaction or re-face attack in aza-Diels-Alder reaction by diene, resulting in predominant products with excellent enantioselectivity (the favorable re-face attack in Diels-Alder reaction or si-face attack in aza-Diels-Alder reaction). A "pocket-like" chiral cavity constructed by chiral catalyst enhanced exo-diastereofacial selectivity significantly by maximizing the overlap of the two indole rings of substrates to avoid unfavorable steric repulsion between indole ring of 3-vinylindole and chiral ligand. Compared with catalytic aza-Diels-Alder reaction of 3-vinylindole and ketimine, the improved catalytic activity of chiral N, N'-dioxide-Ni(II) catalyst in Diels-Alder reaction of 3-vinylindole with methyleneindolinone contributed to the high turnover frequency (TOF = 1.05 x10(6) s(-1)). These results were in good agreement with experimental observations.