Morphology, rheology, and permeability studies were carried out for blends of polyethylene (PE) and polybutylene terephthalate (PBT). The effect of draw ratio at the die exit of the extruder on blend morphology was investigated, and thus isotropic fibrillar morphology with different aspect ratio was obtained. The solvent barrier properties of these blends improve as the aspect ratio of the PET fibrils increases. Rheological experiments showed that these properties can be closely related to the development of a physical network structure of the PET fibrils. Furthermore, the elaboration of a composite polymer system PE/PBT has been made by a second extrusion processing step. This system led to anisotropic oriented PET fibrils in the PE matrix. Thus, random oriented PET fibrils appeared to be an effective way of improving the barrier properties of PE/PBT blends.