Abstract: A model of real sandstones was applied to study the micro-flow characteristics and oil displacement efficiency of the Chang 4+5 reservoirs in the western Nanliang oil field to improve the efficiency of water flooding. The micro-structure was studied with casting thin sections, scanning electron microscopy and mercury injection in order to estimate the influences of reservoir characteristics and micro-flow experiment factors on water flooding efficiency. The results showed that the Chang 4+5 reservoirs were dominated by homogeneous seepage, while finger-type seepage and net-type seepage also existed locally. The homogeneous seepage resulted in the highest final oil saturation and final displacement efficiency. For the internal factors affecting displacement efficiency, heterogeneity played a key role, and reservoir wettability was also important. Permeability was positively related to oil displacement efficiency, while porosity had a weak influence on oil displacement efficiency. Two external factors, water injection pressure and volume multiple, could enhance displacement efficiency, and the influence of pressure was more significant.