Fly ash cenospheres are inexpensive, readily available from coal burning or heavy oil combustion, not an eco-friendly material. If ways can be found to use this, it will serve the twin purposes of facilitating applications for the ash bearing materials and at the same time reduce pollution. One way to achieve this task is to make ash-bearing composites having polymer matrices. The performance of filled polymers is generally determined on the basis of the interface attraction of filler and polymers. Fillers of widely varying particle size and surface characteristics are responsive to the interfacial interactions with the polymers. The present study deals with the effect of particle size (150 mesh, 100 mesh, and 300 mesh) variations in fly ash cenospheres, as a filler with different concentrations (0-40 wt%), on various properties of acrylonitrile butadiene styrene. The mechanical, thermal, and electrical properties of the composite material were evaluated, and the microstructure was investigated through scanning electron microscopy. The smaller particle size showed better properties in comparison with larger particle size. As increasing filler loading, the saturation level is influenced by the agglomeration of filler particles in the polymer matrix. Thus, the performance of polymer filled with fly ash cenosphere composites is the function of the particle size, the dispersion, and the interfacial interaction between the filler particles and the polymer matrix. © The Author(s) 2012.