Nanofluids are a new class of fluids engineered by dispersing nanoparticles in base fluids. The addition of small amount nanoparticles may enhance the thermo-physical properties of the original liquids. In this study, thermal conductivity of pristine and modified multiwalled carbon nanotubes (MWCNT) in water-based fluids was prepared and investigated at various temperatures ranging from 6OC to 45OC. Stable and homogeneous MWCNT nanofluids were successfully produced with an addition of polyvinylpyrrolidone (PVP) as the dispersing agent using physical agitation process. The addition of MWCNT into a fluid leads to the enhancement of its thermal conductivity. The prepared nanofluids, with good fluidity, stability, and high thermal conductivity, is a potential advanced coolant in thermal energy engineering and energy consumption saving.