It has been shown that the free volume fraction at Tg is not a universal parameter for linear polymers of different molecular structure[2]．The reason is that the volume expansion at Tg is partially contributed from the change of the numbers of eonforma-tions of isolated molecular chains due to internal rotation．In this paper，glassy trans- formation was connected with internal rotation of isolated molecular chains，and the relationship between free volume fraction of polymers at Tg and energy E of rotational isomerization of isolated molecular chains was formulated，The values of calculated from the above formula are in good agreement with those published in the literatures．Thus，the method described in this paper call be used to estimate a varameter for the flexibilitv of isolated molecular chains．