Traditional heat transfer fluid has a heat transfer characteristic which is worse than that of the solid although many studies have been conducted to increase heat transfer in fluid. One of the various heat transfer enhancement techniques is to suspend fine metallic or nonmetallic solid powder in traditional fluid. Nanofluid is defined at a new kind of heat transfer fluid containing a very small quantity of nanometer particles that are uniformly and stably suspended in a liquid. In this study, CuNi or CuAg nano particles are used to investigate heat transfer enhancement. Through the molecular dynamic(MD) method, molecular behavior was clarified on an interface to examine the heat transfer mechanism of nanofluid in a molecular dynamics point of view . The result showed that the thermal conductivity of 0.5vol% CuNi nanopowder in ethyleneglycol solution was 124% higher than that of ethyleneglycol solution only. By the numerical analysis for the molecular motions at the liquid-vapor interface, a criterion to distinguish between liquid and vapor was suggested by a potential energy and the number of neighboring molecules. When a molecule moved into a vapor region from a liquid region, the potential energy of a molecule was increased but the number of neighboring molecules was decreased.