The conformational transition behavior around the glass transition temperature has been studied by means of molecular dynamics through a model system, atactic polypropylene in bulk. Various criteria of the conformational transition were examined. A domain at barrier site is defined as the transition state of the conformers and the other conformational states are labeled in code. The transition is accepted when the code is changed. Variation of the width of the domain at barrier distinguishes the deep jumps of the torsion rotations from the shallow jumps that do not reach the well bottom within +/- 20 degrees from the minimum. The behavior of the deep jumps obeys the rate equation of the transition-state theory, being independent of T-g. The shallow jumps were found significant in the characterization of the motion of polymer chains through T-g. This study thus proves that the changes of properties of polymers resulting from the glass transition are predominately mastered by the shallow jumps.