Investigation of the extensional flow behavior and development of constitutive equations for well-characterized H-shaped polymers is important both from an industrial and academic viewpoints. For H-shaped polymer systems, little is known about the extensional flow behavior at constant strain rates. We synthesized H-shaped poly(methyl methacrylate)s (PMMAs) with narrow molecular weight distributions and various ratios of the molecular weight of the backbone, M (b), to that of the arm, M (a), by atom transfer radical polymerization. However, H-1 NMR spectroscopy showed that the resulting PMMA samples contained substantial amounts of linear and star topologies. In spite of the limited purity of the H-shaped PMMAs obtained, the rheological properties in linear viscoelasticity and uniaxial extensional flow revealed interesting results. The zero-shear viscosity of model PMMAs was well described by an exponential relation of the molecular weight. Qualitative agreement was found between the experimental linear-viscoelastic behavior and predictions of the model of McLeish et al. (Macromolecules 32: 6734-6758, 1999) for H-shaped polymers. The elongational flow behavior was also analyzed by the molecular stress function (MSF) model. Surprisingly, the strain-hardening effect for all investigated model PMMAs was weaker than that for pom-pom polystyrene polymers.