Mobile amorphous fraction and crystallinity of uniaxial poly(ethylene terephthalate) (PET) films were quantitated, using temperature modulated differential scanning calorimetry (TMDSC), and differential scanning calorimetry (DSC). Our results show that the crystallinity increases with decreasing drawn temperature and increasing draw ratio. The strain rate also affects the crystallinity. From complex heat capacity measurement, we estimated an average length of Cooperative Rearranging Region (CRR), xi(alpha), a characteristic parameter of the dynamic heterogeneity in glass transition. Despite the changes in the drawing or annealing condition, the xi(alpha) increased linearly with increasing weight fraction of the mobile amorphous phase. Crystal and rigid amorphous PET suppressed the formation of CRR. Finally, we also estimate from extrapolation, xi(alpha) = 3.8 nm, for a perfect amorphous PET. (C) 2013 Elsevier Ltd. All rights reserved.