Dynamic mechanical measurements were conducted for several kinds of ultra-high-strength polyethylene fibers with different methyl branch contents. As is the case with conventional polyethylene materials, UHSPE fibers also exhibit alpha, beta, and gamma-relaxation dispersions. Each relaxation process is the function of both the tensile moduli and the branch contents of UHSPE fibers. It was also found that the gamma-process of UHSPE fibers is dominated mainly by the localized molecular motion in the crystalline part, such as a dislocation mode of crystallite defects, which is very sensitive to the branching content. From the time and temperature superposition of the frequency dispersion experiments, it was found that activation energies for both the alpha 2-process and alpha 3-process increase proportionally to the methyl branch content, while the alpha 1-process is not so affected by the branch content. This result shows that the incorporated branch sites in the crystalline part effectively hinder the chain-to-chain slippage; meanwhile, they have not hindered the slippage at the grain boundary so far, which also enables us to explain the creep improvement of UHSPE fibers through branch incorporation with the same mechanism.