3,4,5-Trihydroxybenzyl acid dimer has been studied by using density functional theory B3LYP method and 6-31G*,6-31G**,6-31+G* basis set.The calculaed results indicate that the title compound has C2 symmetry and exhibits strong hydrogen bonding interaction,and interaction energy is 60～66 kJ/mol after basis set superposition error and zero-point vibration energy correction.There are large red-shifts for the symmetric H-O and C=O stretching vibration frequencies in the 3,4,5-triysroxybenzyl acid dimer due to forming C=O…H-O hydrogen bond.The NBO analysis shows that the charge transfer goes from the lone pairs of the proton acceptor oxygen atom to the σ*（H-O） antibonding orbital of the proton donor,their interaction stabilization energy is 91.59 kJ/mol.
