Abstract: This paper describes the characteristics of pore structures of the ultra - low permeability reservoirs in Wan-gyao block, Ansai oilfield. It studies sensitivity of permeability to effective pressure and sensitivity of permeability hysteresis and value to effective pressure based on permeability and effective pressure experiments of 25 cores. Study shows that permeability changes greatly at low effective pressure and all cores have certain plasticity during repeated change when bedding fissures or microfissures exist Influences of effective pressure and gas slippage effect should be taken into consideration when evaluating reservoirs. Influences of microfissures should be taken into consideration during waterflooding.