A molecular dynamics simulation of two-dimensional Couette flow has been conducted to investigate the influence of fluid density on velocity slip of fluid flow in nanochannel.The effects of fluid density on microscopic structure，velocity distribution and slip length are analyzed and discussed.The results indicate that oscillation distribution of fluid density occurs near the wall owing to the effect of solid surface potential，and a small oscillation and layering as well as a wide homogenous region are observed with the decreasing ratio of fluid-to-solid density.In addition，decreases in the ratio of fluid-to-solid density induces the hardship of the motion state of solid surface transferred to the fluid and hence lead to large loss of energy transport across the fluid-solid interface，which contribute to large velocity slip at the boundary.Accordingly，the slip length increases as the ratio of fluid-to-solid density decreases.