Nanofluids have shown their advantages and potentials in improving heat transfer rates when the nanofluids are applied as working fluids in thermal systems. Researcher groups concentrating on the nanofluids have increased continuously and focused deeply into various fields; theoretically, experimentally and numerically. This review summarized the important published works on nanofluid preparations, properties, experimental and numerical heat transfer behaviors. In the simulations, two main categories were discussed in detail as the single-phase modeling which the combination of nanoparticle and base fluid is considered as a single-phase mixture with steady properties and the two-phase modeling which the nanoparticle properties and behaviors are considered separately from the base fluid properties and behaviors.