Thermal oxidation-induced long chain branching (LCB) during the molding processes for polyolefin copolymer poly(ethylene-co-butene) (PEB) and its blend with another polyolefin copolymer poly(ethylene-co-hexene) (PEH/PEB50/50 blend, denoted as H50) was investigated mainly by rheological measurements. LCB with different levels could be introduced on PEB backbones by changing the molding temperature and/or molding time, which could be sensitively characterized by changes of rheological parameters, that is, storage modulus G' and complex viscosity eta*. Thermal oxidation-induced LCB of PEB in H50 samples could largely influence the phase separation kinetics. Rheological measurements and phase-contrast optical microscope observations coherently indicated that thermal oxidation-induced LCB of PEB more or less retarded the development of phase separation and once it reached a certain level, the reduced chain diffusion even arrested phase separation. The decrease of mass-averaged molecular mass in H50 with high LCB level was ascribed to the reduced hydrodynamic volume. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 119: 530-538, 2011