The structure and energetic of hypocrellin A has been studied theoretically by the B3LYP/6-31G(d) method. In the gas phase, the calculated results predict that the gauche conformation of 'normal' tautomer is dominant in eight isomers; the gauche conformation of 'double' tautomer (GDHA) is the next most stable isomer. The single point energy calculated results indicate that GDHA is dominant in ethanol, acetone or DMSO. Additionally, ground- and excited-state intramolecular hydrogen transfer among eight tautomers of hypocrellin A has been investigated by the B3LYP/6-31G(d)//B3LYP/4-31G and TD B3LYP/ 6-31G(d)//CIS/3-21G method. (C) 2008 Published by Elsevier B.V.