Abstract: Using the fresh oil?bearing core samples, this paper studied the remaining oil micro?distribution or occurrence and the influence of long?term waterflooding on the reservoir pore structures in major reservoir of Laojunmiao oilfleld in Yumen. The study indicates that the remaining oil occurrence in the reservoir is dominated by film or interstitial form, and the pore stucture is the key factor influencing the remaining oil distribution and oil displacement efficiency. After long?term water flooding, the interstitial material and flowing resistance are decreased in the pores with better physical property. For L layer, its remaining oil is still distributed in big pores，while for M layer, its one third of remaining oil is in the small pores or disseminated on rock surface, and hard to flow. The content of movable water in the big pores is directly related to the total water cut in the reservoir