Differences in processability between linear low denisty polyethylene (LLDPE) and low density polyethylene (LDPE) as well as for their blends subjected to extensional flow were assessed in terms of melt strength and elastic parameters from shear flow such as storage modulus (G＇), and first normal stress difference (N-11 - N-22). The results revealed similar synergistic effects for these parameters in the LDPE rich blends. A DSC rest carried out on these LLDPE/LDPE blends when fast cooled disclosed the existence of two phases in the molten blends for the same composition range where the melt strength synergism was observed. The results are discussed in terms of the influence of the different molecular structures of the blend polymers and their miscibility on the measured melt strength. A mechanism is suggested to explain the observed synergistic phenomenon.