The size and concentration of free-volume holes of two kinds of montmorillonite (MMT)/styrene-butadiene rubber (SBR) nanocomposites were investigated by positron annihilation lifetime spectroscopy (PALS). Strong interfacial interaction caused an apparent reduction of the free-volume fraction of rubber probably by depressing the formation of free-volume holes in the interfacial region. Interfacial interaction in MMT/SBR nanocomposites was weaker than that in SBR filled with carbon black.