Present study deals with the effect of variation in melt flow index (MFI) of maleic anhydride-grafted polypropylene (PP-g-MAH) as a polymeric compatibilizing agent on various properties of fly ash (FA)-filled polypropylene (PP) composites. The FA content was varied from 0 to 40 wt%. The effect of polymeric compatibilizing agents with different MFI and very high maleic anhydride (MAH) content on interfacial adhesion between filler and matrix and filler dispersion were studied. The mechanical and thermal properties of the composite material were evaluated, and the microstructure was investigated through scanning electron microscopy. The values of yield stress and breaking strength of compatibilized PP/PP-g-MAH/FA-based composites showed higher values compared to that of untreated FA-filled PP composites at corresponding filler content. Incorporation of FA into PP led to stiffer materials, as tensile modulus increased significantly. Tensile and impact properties varied with varying molecular weight of PP in PP-g-MAH and are essentially decided by wettability of the filler. It is also found that heat-deflection temperature and vicat softening point improved with the addition of FA filler. The use of PP-g-MAH as polymeric coupling agent provides improvement in mechanical and thermal properties of filled polymers. The higher effect of compatibilization is obtained using high-molecular-weight PP in PP-g-MAH as a polymeric compatibilizing agent and low-molecular-weight PP in PP-g-MAH resulted in better dispersion of FA in PP matrix. The overall results showed that FA dispersion and interfacial adhesion are greatly affected by the kind of PP-g-MAH. © The Author(s) 2013.