The conformational transition behavior of polymer in the amorphous state has been investigated through molecular dynamics simulations across the glass transition temperature (T-g). We find that the conformational transition, a localized and short time dynamics feature, crosses over different barrier heights when the system transforms from the molten state into the glass state and the barrier height in the glass state is markedly lower than that above T-g. In addition to the overall transition behavior, the specific transitions between the rotational isomeric states (RIS) g(+), t(-), t(+) and g(-) are also investigated in detail. The populations of these specific transitions undergo considerable changes when the temperature decreases; meanwhile, the larger transition rates of the ending torsions get diminished. Besides the rate, the rotation degrees of the dihedrals during the transitions also change their distributions tremendously, through T-g, below which most of the larger transition angles (50-100 degrees) were inhibited remaining those sharply around 30 degrees. This possibly explains why below T-g the conformational transition process has a lower effective barrier. (C) 2009 Elsevier Ltd. All rights reserved.