The present article studies the effect of treatment of 0.5, 1.5, and 2.5% titanate-coupling agent (LICA 38) on various properties of fly ash–filled polypropylene (PP) composites. The fly ash content varied from 0 to 30 wt%. The mechanical and thermal properties of the composite material were evaluated, and microstructure investigated through scanning electron microscopy. Experimental results were compared with various existing models. Experimental data for tensile yield strength showed good fit to the existing models. Adhesion parameter or interfacial interaction was also evaluated though Pukanszky model. The values of yield stress and breaking strength of treated fly ash–based composites showed higher values compared to that of untreated fly ash–filled PP composites at corresponding filler content. The overall mechanical properties of fly ash–filled composites are essentially decided by wettability of the filler. It is also found that Vicat softening point improved with the addition of fly ash filler. Morphological studies of the tensile fracture surfaces of the composites revealed that the presence of titanate-coupling agent increased the interfacial interaction between fly ash and PP. It also improved the dispersion of fly ash in PP matrix. Thus, the treatment resulted in improvement in mechanical and thermal properties of the composites as compared to untreated fly ash–filled composites. The overall results showed that fly ash dispersion and interfacial adhesion are greatly affected by the amount of the coupling agent.