In order to study the rule ofwaterflooding in ultra-lowpermeability reservoirs, 57 pieces of cores from3 well fields in Ordos Basin are selected to carry out experiment on waterflooding, and the relations ofwaterflooding efficiency with permeability, water injection multiple, water injection pressure and so on are studied. The selected cores mainly are ultra-lowpermeability that covers with Chang 4+5, Chang 6 and Chang 8 reservoirs. The results indicate that waterflooding efficiency increases with permeability increasing and quickly increases in lower permeability reach. It is the same situation with water injection multiple increasing, but there is lower increasing range in stage of high water-cut and no increasing until water-cut to 100%. At the same waterflooding speed, the higher the permeability is, the higher the waterflooding efficiency will be, but the waterflooding efficiency will be altering when at different waterflooding speed. There exists a best speed, and the higher the permeability is, the higher the speed is. The waterflooding efficiency generally decreases in water-free stage and increases in water-bearing stage with water injection pressure increasing and it gradually decreases when the layer gets a best pressure. The stronger the heterogeneity is, the lower the waterflooding efficiencywill be.