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The POM−POM architecture is the simplest yet defined branched architecture, showing both strain hardening in elongation and strain softening in shear. The molecular structure consists of q side chains at each end of a backbone segment. To study the rheological and mechanical properties, we synthesized low-disperse POM−POM-shaped polystyrenes (PS) with welldefined molecular properties via anionic polymerization and grafting-onto method. All samples had a backbone with a weightaverage molecular weight of M w,b ≅ 100 kg mol −1 and approximately similar numbers of side chains per star q = 11−14. We varied the side chain length systematically from unentangled up to highly entangled side chains (M w,a = 9−300 kg mol −1 , 0.5−18 entanglements). The POM−POMs having M w,a ≈ 3M e ≈ M c have a maximum decrease in zero-shear viscosity η 0 of over 3 decades compared to linear PS with the same molecular weight, together with the highest strain hardening factor of SHF = 43. Moreover, POM−POMs having M w,a > 5M e displayed enhanced mechanical fatigue resistance beyond those of linear, ultrahigh-molecularweight PS, by up to a factor of 10.