The changes of the mobility of rubber molecules for carbon black-filled EPR composites during mixing process in an internal mixer have been elucidated from T2 measurement using pulsed nuclear magnetic resonance (NMR). The thickness of the tightly bound region which is formed by the direct interaction with carbon black was stabilized with mixing but the restriction of the molecular mobility increased with further mixing. The amount of the loosely bound region increased considerably throughout the mixing, indicating that this region is formed by entanglement and crosslinking of the molecules. T2 of the mobile region which is a predominant part in the composites is independent of the mixing time. EPR has the thinner tightly bound shell and longer T2s compared with EPDM, showing the effect of unsaturated bond in EPDM which can become a strong bond site with carbon black.