To withstand harsh conditions and have a moderate strength, it is desirable to use natural rubber for base isolators. In addition, previous studies have measured the magnetorheological (MR) effect under low-strain range, mostly within 10%. In the reality, it is necessary to evaluate the performance under large-strain range for base isolators. In this study, material properties of natural rubber-based MREs with various mixing ratios were evaluated under large-strain range (similar to 100%). In the first step, MREs with various iron ratios were fabricated and evaluated to observe the MR effect according to the ratio and arrangement of iron powder. As a result, the highest MR effect (22.0% at 100% strain) and damping ratio (10.29%) were observed in the sample with 35% iron ratio, and the MR effect of the isotropic and the anisotropic MRE did not show significant difference under large-strain (50 similar to 100%). In the second step, MRE samples containing the optimum iron ratio (investigated in the first step) and various mixing ratios of carbon black and naphthenic oil were prepared. As a result, the MRE containing 60phr of carbon black and 40phr of naphthenic oil had the highest MR effect (33.8% at 100% strain). Compared to the case without additives, it showed an obvious improvement.