Abstract: The two-phase flow characteristics of water flow displaced by gas at nanometer scale have a wide application in the development of nano-fluidic chips and shale gas. In this paper, experiments of water flow displaced by gas and single-phase flow was carried out to study the flow characteristics of water flow displaced by gas at nanoscale of alumina membranes with diameters of 292.8 nm, 206.2 nm, 89.2 nm, 67.0 nm and 26.1 nm. The experiments show that there are three flow stages for water flow displaced by gas at nanoscale: firstly, the flow rate increases slowly with increasing the driving pressure in the first stage, and the gas flux is decreased about one order of magnitude when compared with gas single-phase flow; secondly, gas flux increases rapidly with increasing the driving pressure as water in nano arrays are largely driven out in the second stage; thirdly, gas flow characteristics are consistent with the single-phase flow when water in nano arrays are all displaced out in the third stage. The analysis results show that due to the “pinning" effect of capillary pressure at the gas-liquid interface and the interaction of solid-liquid interface, gas flux increase slowly in the first stage of water flow displaced by gas. And the larger driving pressure is required to increase the gas flux in nano arrays, which is the main reason of the nonlinear flow characteristics. Once the “pinning" effect is destroyed, the gas will enter the pipeline to promote the movement of the interface, which will change the capillary curvature between capillary column and liquid column. As a result, the capillary force reduces rapidly with the increase of displacing pressure and gas flux will increase sharply, which is the main reason of the sudden change of water flow displaced by gas in the second stage.