In this study, the effect of oil fly ash (OFA), a by-product of oil fuel power plants, on the rheological and morphological behavior of low-density polyethylene (LDPE) is investigated. As received and acid-functionalized OFA (COOH-OFA) are used to examine the effect of surface modification of OFA on polymer–filler composites. LDPE/OFA composites were prepared by melt mixing with filler loading in the range 1–10 wt %. The results are compared with pure LDPE. The effect of polyethylene-grafted-maleic anhydride (PE-g-MA) as a compatibilizer was also studied. Both viscous and elastic properties of composites increased with OFA loading especially at low frequency. The surface modification of OFA has influenced the properties of OFA. As-received OFA showed some agglomeration at high loading that resulted in two-phase system as described by scanning electron microscopy (SEM) and Cole–Cole plot. Field emission-SEM (FE-SEM) images showed improvement in the dispersion of COOH-LDPE/OFA composites. In addition, the surface modification reduced the size of agglomeration. In general, the COOH modification of OFA improved both the dispersion and rheological properties of OFA. With chemical modification, the concentration of the filler can be increased to 10% without compromising the properties of the composites. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011.