Abstract: In order to achieve cost-effective development of the ultra-low permeability reservoirs in Dongfenggang Oilfield, micro-pore structure and seepage characteristics of the reservoirs were studied. Taking the upper section of Es4 Formation as an example, detailed pore structure properties such as throat radius, pore radius, ratio of pore radius to throat radius and capillary pressure distribution were obtained through constant speed mercury injection. Then, the seepage characteristic properties of the ultra-low permeability reservoirs were acquired from relative permeability experiments of water displacing oil with the unsteady state technique of core samples under 70 ℃. Experiments showed that the larger the throat radius distribution range is, the larger the peak value would be and that pore radius distribution presented no significant difference; in fact, pore to throat radius ratio depends on the permeability and the displacement pressure is very high for cores with ultra-low permeability due to their very small radius of connected throats. Thus it is challenging to produce ultra-low permeability reservoirs. In addition, the average irreducible water saturation of the reservoir is high, which would result in oil-water two phase flow area reduction as permeability increase. The results of the experimental study indicated that there is no strong correlation between the oil displacement efficiency and permeability. Most of normal water phase relative permeability curves fit reservoirs with large throat radius and poor connectivity. In contrast, the linear type water phase relative permeability curves fit reservoirs with small throat radius but good connectivity. Therefore, it is of great significance to reveal the micro pore structure in order to create the best possible development strategy.