The elaboration of an in-situ composite consisting of ethylene vinyl acetate copolymer (EVA) and polybutylenetherephtalate (PBT) was investigated. An interchain chemical reaction during processing operations was used to generate a grafted PBT-g-EVA copolymer at the interface of the two polymers. Composites with either nodular or fibrillar morphologies were obtained. Fibrillar morphology was achieved by stretching the extruded blend at the exit of the die. Elongational properties of such composites were investigated by using the fiber wind up technique. The influence of the aspect ratio of PET solid inclusions on the elongational viscosity was discussed for a series of EVA/PBT composites. Clearly, the strain hardening/ softening properties were found to be strongly affected by the aspect ratio. Increasing the aspect ratio of PBT solid particles made the strain hardening weaker and even gave strain-softening. This behavior is emphasized by the presence of the grafted PBT-g-EVA copolymer. The results suggest that strain-hardening or softening of such composites is correlated to the homogeneity of the flow at EVA-PBT interface: the elongational flow is disrupted in the interphase region so that the deformation around the particle is not homogeneous.