The long-chain branched ethylene-tetrafluoroethylene co-polymer (B-ETFE) was synthesized by radical polymerization using ethylene, tetrafluoroethylene, termonomer, and a very small amount of divinyl monomer. We studied melt rheology under shear and elongational flow, and foamability for B-ETFE, comparing with conventional linear ETFE. Addition of a small amount of the divinyl monomer had a considerable impact on the melt rheology of ETFE and its blends. The observed non-linear elongational behavior and emergence of long relaxation time components can be explained by long-chain branching in the modified ETFE. B-ETFE and its blends showed excellent foaming processability: uniform and smaller cell sizes and much higher cell number densities than those of linear ETFE. To the best of our knowledge, this is the first report of the enhancement of the melt rheology of ETFE from the viewpoint of modification of its molecular architecture.