The comparative studies for the effect of vinyl‐modified silica (VMS) and raw silica (RS) particles on the cell structure, insulation (dielectric and thermal transport) properties, and thermal stability of thermoplastic PMMA‐silica nanocomposite (PSN) foams are described. The VMS particles were synthesized by the conventional acid‐catalyzed sol‐gel reactions of tetraethyl orthosilicate (TEOS) in the presence of 3‐(trimethoxysilyl)propyl methacrylate (MSMA) molecules. The as‐prepared VMS particles were then characterized through fourier transform infrared (FTIR), solid‐state ¹³C‐nuclear magnetic resonance (¹³C‐NMR) and ²⁹Si‐NMR spectroscopy. Subsequently, the PSN materials were prepared via in‐situ bulk polymerization. The dispersion of silica particles in PMMA matrix was observed by transmission electron microscopy (TEM) studies. Gel permeation chromatography (GPC) was used to determine the molecular weights of as‐prepared samples. The PSN materials were used to produce foams by a batch process in an autoclave using nitrogen as foaming agent. The effect of VMS and RS particles on the cell structure, insulation properties and thermal stability of PSN foams were investigated by scanning electron microscopy (SEM), LCR meter, Transient plane source (TPS) technique and thermal gravimetric analysis (TGA), respectively. The better dispersion capability of VMS particles in PSN foams was found to lead enhanced nucleation efficiency, thermal stability and decreased dielectric constant (ε′), dielectric loss (ε″) and thermal conductivity (k). POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers