Poly(methyl methacrylate) (PMMA) and sodium montmorillonite (Na-MMT) were melt-blended with 1-dodecyl-3-methylimidazolium hexafluorophosphate (C(12)mimPF(6)) to prepare nanocomposites containing well-dispersed MMT 'layers in nanoscale and freely existed excess C(12)mimPF(6). A synergistic effect of MMT and C(12)mimPF(6) on the foaming behavior of nanocomposites was revealed in this study. At the mild conditions of 17 MPa and 35 degrees C, neat PMMA and PMMA/C(12)mimPF(6) and PMMA/Na-MMT samples were not foamed, while cells were induced in nanocomposites containing a certain amount of free C(12)mimPF(6). At 25 MPa/90 degrees C, foamed nanocomposites exhibited higher nucleation efficiency and cell density, and smaller cell sizes with narrower size distribution than that of neat PMMA and those binary samples. The results indicated that the MMT nanoparticles produced heterogeneous nucleation, and increased the modulus of polymer matrix to depress cell coalescence. The presence of free C(12)mimPF(6) decreased the energy barrier for cell nucleation and facilitated the dispersion of MMT nanoparticles via in situ organic modification.