Various thermal histories were utilized to generate samples with the same crystalline microstructure (i.e. degree of crystallinity, supermolecular structure, tie molecule density and lamellar thickness) for linear low-density polyethylenes (LLDPEs) with the same molecular weight, molecular weight distribution and branch frequency but different branch length. The static fatigue properties were found to improve with decreasing applied load for samples with the same type of short-chain branches. The failure time of static fatigue (t(f)) was found to increase dramatically as the branch length increased. An equation was used to predict tf from the stress, the branch length and other material parameters. In addition, the initial growth rate of the crack opening displacement and the time required to reach the critical opening displacement at the notch roots of the specimens were observed to decrease and increase, respectively, with increasing branch length. This dramatic improvement in static fatigue properties is attributed to the increasing sliding resistance of the polymer chains through the crystal and through entanglements in the amorphous region as the branch length of LLDPEs increases.