A theoretical study with quantum chemistry calculations was performed to investigate highly efficient reduction of CO2 to formic acid using Al under mild hydrothermal conditions. All the structures of the reactants, products and transition states were optimized by B3LYP functional using the extended 6-311 + G(3df,2p) basis set. The formation of HA1OH species, an intermediates in the reaction of Al with H2O, was calculated firstly. The formation of HCOO- was simulated subsequently. Our theoretical results showed that Al-H is a key intermediate species in the highly efficient reduction of CO2 to formic acid using Al under mild hydrothermal conditions, which demonstrated that the formation of formic acid is through an SN2-like mechanism. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.