The hybrid calculations with ONIOM(B3LYP/6-31G*:AM1) method were carried out on the tautomerization reaction of formamide to formamidic acid in the microcontainer-encapsulated state.The free-state tautomerization process was also investigated with B3LYP/6-31G**//B3LYP/6-31G* method for the purpose of comparison.Bare tautomerization,H2O-assisted(single-H2O or multiple-H2O) and self-assisted mechanisms were all taken into consideration for the encapsulated state.The results show that only bare tautomerization and single-H2O catalysis mechanisms are possible to the encapsulated for-mamide tautomerization owing to the container’s size confinement effect.Geometrical changes in the complexed container and guest molecules are discussed to deeply understand the complex’s structural properties.The bare tautomerization barrier in the encapsulated state increases by 23.826 kJ/mol,ac-counting for 12% of the corresponding total energy barrier in the free state,and the increased values for the single-H2O catalysis process are 12.958 kJ/mol,accounting for 16% of the corresponding total energy barrier,respectively.This finding suggests that the encapsulation can make the tautomerization process slightly difficult. The hybrid calculations with ONIOM (B3LYP / 6-31G *: AM1) method were carried out on the tautomerization reaction of formamide to formamidic acid in the microcontainer-encapsulated state. The free-state tautomerization process was also investigated with B3LYP / 6-31G ** // B3LYP / 6-31G * method for the purpose of comparison. Bare tautomerization, H2O-assisted (single-H2O or multiple-H2O) and self-assisted mechanisms were all taken into consideration for the encapsulated state.The results show that only bare tautomerization and single-H2O catalysis mechanisms are possible to the encapsulated for-mamide tautomerization due to the container’s size confinement effect. Geometrical changes in the complexed container and guest molecules are discussed to deeply understand the complex’s structural properties. The bare tautomerization barrier in the encapsulated state increases by 23.826 kJ / mol, ac-counting for 12% of the corresponding total energy barrier in the free state, and the increased values ​​for the singl e-H2O catalysis processes are 12.958 kJ / mol, accounting for 16% of the corresponding total energy barrier, respectively. This finding suggests that the encapsulation can make the tautomerization process slightly difficult.