The melt rheology of polypropylene (PP) filled with fly ash (FA) before and after treatment with coconut water (CW) was studied for different concentration of the filler. The fly ash after coconut water treatment clearly showed additional peaks in the infrared (IR) spectra corresponding to the hydroxyl and carbonyl groups indicating good adsorption of CW on FA. The X‐ray diffraction of melt compounded PP filled with CW‐treated (CWT) FA showed large reduction of the main silica peak of FA and considerable broadening of Mullite and hematite peaks suggesting formation of fine particles by this treatment. Scanning electron microscopy (SEM) confirmed the drastic reduction of particle size in these composites. The melt rheological studies for these composites indicated considerable increase in viscosity at low filler loading for CW treated FA. The concentration dependence of melt viscosity did not follow any of the theoretical equations suggested in literature. Although, the behavior was similar to nanoparticle‐filled polymers, there were some differences especially above the critical concentration of 4.5% by volume. The frequency dependence of storage and loss modulus indicated crossover point clearly, which was greatly affected by CW treatment. The Cole–Cole plots of real and imaginary part of melt viscosity brought out the broad distribution of relaxation time for the CW treated FA. The CW treated FA melt compounded with PP gave rise to nanocomposites with uniform dispersion. However, above 4.5% by volume, there appears to be agglomerate formation along with a thin interfacial layer, which assists the melt flow even at high filler loading.