The reaction mechanism of the one-way isomerization of 2'-hydroxychalcone (2HC) was studied by calculating the potential energy surfaces of the twist of the C=C double bond and the intramolecular hydrogen atom transfer in the excited triplet state by the UB3LYP/6-31G** level. The calculations reveal the following. The potential energy curve of the triplet state between the keto forms (trans-2HC and cis-2HC) is similar to that of stilbene, which exhibits mutual isomerization. On the other hand, the potential energy curve between the enol forms (trans-2HC' and cis-2HC') is similar to that of styrylanthracene, which exhibits one-way isomerization from the cis to trans isomer. Although the hydrogen atom transfer has an energy barrier (<2 kcal mol(-1)), the relative energy of the transition state with zero-point correction is lower than that of trans-2HC. Electronic spins localize around the C=C double bond in the intermediate states (p-2HC and p-2HC') of isomerization. During the hydrogen atom transfer, electronic spins shift from the phenyl group to the 2-hydroxyphenyl moiety. The electronic charge on hydrogen-bonded proton does not change significantly during the hydrogen atom transfer process.