ABSTRACTFly ash (FA) based polymer composites are assuming increasing importance because of its potentiality, fine particle size and plenty availability of FA. FA is mainly a mixture of inorganic metal oxides such as SiO2, Al2O3, Fe2O3, CaO, MgO, Na2O, TiO2, and so forth. This article highlights the results of the various modifications onto the HDPE‐FA/nano structured FA (NFA) composites. When FA and NFA are melt blended with HDPE it gives rise to improved flexural properties only. Further modifications, that is, Maleic anhydride (MA) grafting of the matrix, electron beam irradiation of the composite and irradiation of the FA/NFA studied separately to find their impact on the detail properties of the composite. Of the three modifications implemented the electron beam irradiation of HDPE‐FA/NFA composite yielded excellent physico‐mechanical, thermal and dynamic mechanical properties. Fracture surface analysis of the HDPE, unmodified and modified FA/NFA composites studied employing SEM correlated well with the physico‐mechanical properties. The results prove that FA is valuable reinforcing filler for HDPE and its size reduction to nano level is a more effective criterion for its future use. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 4558–4567, 2013