Nano?uids are promising intermediate for heat transfer systems. Understanding thermal traort properties of these fluids helps us to understand and implement them in industrial application and scientific studies. Many theories have been provided to predict the behavior and properties of nanofluids, including their thermal conductivity. In these studies, there are many contradictions. This implies that thermal traot in nanofluids faces many challenges. Molecular dynamics simulation method is an appropriate way to simulate nano scale interactions in nanofluids. In this study, the problem of thermal conductivity of nanofluids and the mechanisms involved in this issue, is under consideration. A parametric study on the thermal conductivity of nanofluids and the mechanisms for determining the thermal conductivity is considered here . The studied parameters are: nanoparticle radius and volume percentage and type, nanoparticle interaction strengths between the fluid and solid atom and the system temperature. The results obtained from equilibrium molecular dynamics method for a nanofluid which its nanoparticles are uniformly dispersed, do not show any significant enhancement in thermal conductivity. Heat flux autocorrelation function analysis shows that, unlike the theories which state that the Brownian motion of nanoparticles and micro convection induced in the fluid atoms or atomic layering at liquid-solid interface, are the dominant mechanism of heat transfer in nanofluids, thermal conductivity of nanofluids is more influenced by collisions between the atoms in the environment. Keywords: Nanofluids, Thermal Conductivity, Molecular Dynamics