Nanocomposites based on NR/EPDM/Cloisite 15A have been prepared via a vulcanization process to investigate the effect of nanoclay content and rubber composition on the properties of samples. The prepared nanocomposite samples are characterized by several techniques. The experimental results of X-ray diffraction and transmission electron microscopy showed the widening of distance between the silicate layers, penetration of polymer chains into silicate layers, and formation of an intercalated and partially exfoliated structure. Results indicated that addition of nanoclay could improve the mechanical strength of the rubber. Also increase of nanoclay and EPDM content were found to improve the thermal aging of the samples and also the performance of the rubbers under compression. Nanoclay also reduced the hysteresis and heat build up of the rubber compounds. Moreover, to investigate the stressstrain behavior of NR/EPDM nanocomposites using constitutive models, applicability and best fit of five different hyperelastic constitutive models including second order Polynominal, Yeoh, third order Ogden, Marlow, and Arruda-Boyce against experimental data for NR/EPDM/nanoclay blends were studied. Due to the different amounts of nanoclay (1, 3, 5, and 7 wt %) and different NR/EPDM compositions (100/0), (75/25), (50/50), (25/75), and (0/100), different experimentally measured data of samples were compared with those predicted by the mathematical models previously mentioned. Results showed that third order Ogden and Marlow models can be used for different amounts of nanoclay and rubber compositions, whereas Arruda-Boyce and Yeoh models show more deviation from experimental data with a reduction in nanoclay and increase in EPDM content. (C) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013