Abstract With respect to stress-birefringence, isomerized natural rubber behaves the same as natural rubber at low elongations, but at high elongation levels the natural rubber sample exhibits higher stress-hysteresis accompanied by higher mechanical loss energy. Therefore, it appears that at low elongation level the viscoelastic rate process governing the stress-strain hysteresis is not sensitive to structural imperfection (within the range of cis-1,4 content studied here), but the crystallization process at higher elongation is strongly dependent on the cis content. This is in agreement with the conclusion made by Scott and co-workers from their dynamic measurements on synthetic poly(isoprene)s. The difference in cis-content between Natsyn and natural rubber and the effect on crystallizability explains the lower hysteresis on the stress-birefringence plot for Natsyn. Similar reasoning explains the stress-birefringence difference between the A.C. rubber and Natsyn. The data, however, shows that Natsyn has the lowest amount of loss energy of the three samples (except at very high input energy >4.5). This difference might be due to effects of physical entanglements or gels which may differ. In addition, the difference in microstructure contributes to the difference in extent of strain-induced crystallization. Since the viscoelastic rate process governing the stress-strain behavior and the strain-induced crystallization are associated with molecular motion, they may not be independent.