This paper discusses the problem of interlaminar delamination in composite materials, with particular attention focused on the important role played by the resin properties. A Double Cantilever Beam (DCB) specimen has been modelled for finite element analysis assuming that the crack is situated inside a thin pure resin layer, similarly to the models proposed by Ozdil and Crews. A test program on the epoxy resin EPON 828, a typical resin commonly used for advanced composite systems for aerospace applications, has been carried out, in order to evaluate the mechanical properties; the results have shown a non-linear elastic behaviour without hysteresis effect. The non-linear relationship has been introduced in the bidimensional finite element model for the DCB specimen analysis. The stress and strain fields are used for the evaluation of the Energy Release Rate by means of a numerical model, based on an energetic approach such as the J-integral of Rice. The results show a considerable dependence on the constitutive relationship and give the opportunity of a more accurate evaluation of the toughness for experimental results from DCB specimen.