The influence of the mosaic block structure of bulk-crystallized high-density polyethylene upon deformation within the elastic limit and upon fatigue phenomenon has been extensively studied in the temperature range of the α relaxation mechanism. The magnitude of elastic limit, estimated from two models involving a tilt sliding mechanism and a simple tension mechanism, was compared with experimental results obtained using a tensile tester and a dynamic viscoelastometer at various temperatures of measurement. The two models for onset of deformation are applied on the assumption that the intermosaic block region is selectively affected by strain without any structural change occurring in the mosaic block crystalline core. Also, dynamic mechanical properties were obtained on specimens subject to an increasing number of fatigue cycles. The variation of E'1, max, attributed to deformation processes of the intermosaic block region, is strongly affected by slight elongation and by fatigue cycling of the sample. A direct observation of the mosaic block structure is made with the electron microscope using a detachment replica of the surface of the bulk crystallized material, that has been elongated by 15% at 55°C in biaxial directions.