PolymethylMethacrylate (PMMA) composites with different weight content of organic rectorite (OREC) are prepared via melt blending. The results of X-ray diffraction (XRD) show that the layer distance of OREC is much larger than that of the pristine rectorite. SEM images reveal that the OREC filler has been well dispersed in matrix. With the addition of OREC, Bending modulus increased obviously. Ortho-positronium (o-Ps) lifetimes show that the free volume sizes of PMMA/OREC composites are larger than those of the PMMA matrix. The relationship between the atomic-scale free volume size and mechanical properties is preliminarily discussed in this work.