The nanofiller dispersion greatly influences the properties of nanocomposites. Herein an ionic liquid, 1‐ethyl‐3‐methylimidazolium tetrafluoroborate, is introduced in poly(methyl methacrylate) (PMMA) nanocomposites for mediating dispersion of silica filler. As elucidated by rheology, the ionic liquid is able to improve the degree of retardation of chains diffusion induced by silica although it eliminates the elevation of glass transition. Simultaneously, the ionic liquid incorporated can improve thermal and thermal‐oxidative stability of PMMA due to trapping of radicals and blocking oxygen penetration. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 48007.