The origin of methyl internal rotational barrier in thioacetone has been studied by relaxation effect, natural bond orbital analysis and Pauli exchange interactions. Fully relaxed rotational model, reflecting the true barrier, shows mainly the lengthening of C-carb-C-me bond, opening of CmelCcarbCme2 angle and simultaneous wagging of sulfur atom. Calculation shows that the nuclear electron attraction term (Delta V-ne) is barrier forming and the principal barrier forming term originates from both the C-carb-S(sigma) orbital and lone pair n-orbital. The opening of CmelCcarbCme2 angle and lengthening of C-carb-C-me and C-carb-(sigma) bonds are found to be associated with steric effect and charge transfer interaction between bonding and antilbonding orbitals, respectively. (c) 2006 Elsevier B.V. All rights reserved.