The strain-induced phenomena in cured amorphous EPDM and semicrystalline ethane-octene copolymer (m-PE) were studied using H-1 T-2 NMR relaxation. Samples studied differed with regard to network structure; i.e., EPDM had a larger density of chemical cross-links, whereas the total network density in m-PE was significantly larger because of chain anchoring to crystallites. The effect of these differences on elastic properties was compared. A compression unit was developed for these experiments. This device made it possible to perform NMR experiments under fixed uniaxial compression and to record applied force. Despite the larger total network density in m-PE, the orientation of elastic chains under compression in EPDM is significantly greater, suggesting that the density of chemical cross-links largely determines the elastic behavior. It appears that crystallites in m-PE rearrange upon compression without a change in the crystallinity. It can be concluded from this study that an increase in the density of chemical cross-links is required to improve the compression set of m-PE.