The relaxation behavior of high density polyethylene functionalized with diethylmaleate has been determined by dynamic mechanical method and thermally stimulated depolarization technique. The transitions observed have been related to the structural parameters as well as the alpha-olefin comonomer content and degree of grafting, and compared with previous TSDC results on LLPE functionalized with the same polar group. The lowest relaxation is composed of a sub-gamma and a gamma mode assigned to localized motions of the polar groups and to the glass transition (at 137 K), respectively. The beta-relaxation intensity grows with the grafting degree in agreement with its assignment to motions taking place in the interfacial regions. The behavior of the alpha-mode with different content of diethylmaleate has confirmed its dependence with the crystallite size.