The values of the fragility index of polycarbonate/poly(methylmetacrylate) multilayer films have been determined for materials having thicknesses varying from micro- to nano-scales. The influence of the layer thickness reduction down to 12 nm has been shown to have very different influence on the glass transition parameters of the two polymers. Polycarbonate has exhibited a huge sensitivity to such geometric scale modifications while no modification has been observed for PMMA. A large decrease of the fragility index m, has been observed for PC. This modification is proportional to length scale variations of molecular cooperative motions at T-g, xi(T-g). In regard to the molecular structure of each polymer, these results show that this correlation between fragility index and xi(T-g) is mainly associated to modifications of intermolecular interactions and local density. These material parameters are influencing the activation volume Delta V-# which plays a role on the modification of the fragility index. (C) 2014 Elsevier Ltd. All rights reserved.