The application of poly(ethylene terephthalate) (PET) in extrusion blow molding is limited by its low extensional viscosity, which is related to the low shear viscosity. The suitability of blends of PET/high-density polyethylene (HDPE)/compatibilizer for blow molding is investigated. The rheology of the blends was studied in the context of extrusion blow molding. A digitized video technique is presented for measurement of parison sag and the extent of sag is related to blend composition and processing parameters. Among the blends investigated, blends with a PET content of 70% or above exhibited a high degree of sag. Bottles were blown from the other blends and tested for mechanical properties. On increasing HDPE content, the impact energy of the bottle increases while the bending modulus decreases. A significant increase was noticed in the environmental stress cracking resistance of blends with 20% PET over pure HDPE.