Molecular dynamics simulation has been adopted in investigation of different glass transition behaviors of the bulk and the freestanding thin films (about 8 nm) of atactic polypropylene (a-PP). For characterization of glass transition temperature (T-g) of above systems, both the specific volume and the local conformational transition rate of the systems were examined. The T-g characterization from the local conformational transition rate of polymer chains was recently developed. Our simulation results show that the films have lower T-g than the bulk, and the descent of T-g is in a range of 30 similar to 10 K. These are consistent with experiments. a-PP chain consists of meso-dyad and racemic-dyad, the above results are obtained from both dyads. Individual contribution of the stereoregular dyads to the conformational transition rate was further studied. It was revealed that T-g obtained singly from the meso-dyads was almost the same with T-g individually from the racemic-dyads, although the racemic-dyads have much lower transition rate and higher transition barrier than the meso-dyads. In the present study, the reason for the thin films having lower T-g than the bulk is attributed to the novel behavior that the films have lower transition rate and higher barrier than the bulk when it is below T-g. Such behavior was discussed according to the "coupling rotation" of the dihedrals, which depends on the increase of free volume and the inhibition of kinetic energy.