High-density polyethylene grafted isotactic polypropylene (PP-g-HDPE) was prepared by the imidization reaction between maleic anhydride grafted polyethylene and amine-grafted polypropylene in a xylene solution. The branch density was adjusted by changes in the molar ratio between maleic anhydride and primary amine groups. Dynamic rheology tests were conducted to compare the rheological properties of linear polyolefins and long-chain-branched polyolefins. The effects of the density of long-chain branches on the rheological properties were also investigated. It was found that long-chain-branched hybrid polyolefins had a higher storage modulus at a low frequency, a higher zero shear viscosity, a reduced phase angle, enhanced shear sensitivities, and a longer relaxation time. As the branch density was increased, the characteristics of the long-chain-branched Structure became profounder. The flow activation energy of PP-g-HDPE was lower than that of neat maleic anhydride grafted polypropylene (PP-g-MAH) because of the lower flow activation energy of maleic anhydride grafted high-density polyethylene (HDPE-g-MAH). However, the flow activation energy of PP-g-HDPE was higher than that of PP-g-MAH/HDPE-g-MAH blends because of the presence of long-chain branches. (c) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 2553-2561, 2009