In recent years, the addition of nanoparticles to fluids and polymers has been used as a rheological properties modification. PVA and TiO2 nanoparticles aqueous composite nanofluids were prepared through the use of ultrasonic vibration. In fact, ultrasonic vibration is an advantageous method for nanoparticles dispersion. The preparation method prevents the reduction of polymer’s molecular weight. TiO2 nanoparticles with different concentrations were employed to investigate the rheological characteristics of composite nanofluids. Rheological characteristics of base fluids and composite nanofluids measured at different temperatures. Based on the results, all composite nanofluids, as well as base fluids, exhibit non-Newtonian Behavior, the rheological characteristics of composite nanofluids following Herschel-Bulkley model. Additionally, model parameters are functions of temperature, PVA, and TiO2 nanoparticle concentrations. Also, two-way interactions among temperature, PVA, and TiO2 nanoparticle concentrations affect flow index and consistency index of Herschel-Bulkley model

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