Short branch (methyl branch) effects on the creep properties of the ultra-high strength polyethylene fibers were investigated. The temperature and the stress dependence of creep rates of several high-strength polyethylene fibers having different branch contents, which were prepared by blending of two polymers of highly and less branching, was evaluated according to a model described by Ward and Wilding, and the activation energies and the activation volumes were calculated in terms of their methyl branch contents and tensile moduli. The creep rates of ultra high strength fibers are strongly influenced by their methyl branch contents. The typical branching sample with ca.6 CH3 units per 1000 CH2 units shows ca. 1/20 lower creep rate than that of the less branching (1.0 CH3 per 1000 CH2) fiber sample at the room temperature. The activation energies of creep rate obtained by those highly branching samples are higher than those of lesser branching samples; the difference is nearly proportional to their branch contents. Wide-angle x-ray diffraction results showed that the dimension of a-axis of unit cell increases in proportion to their branch contents. These results imply that the creep mechanism of ultra-high strength polyethylene fibers is dominated by chain slippage in the crystalline part, and also imply that some amount of methyl branch sites can be incorporated in the crystalline part in proportion to the branch content and those incorporated branch sites hinder the slippage motion of molecular chains in crystalline part, which results in the extreme lower creep rate.