Abstract: Oil displacement by imbibition is an effective way of oil recovery in fractured reservoirs. However, there is not a unified recognition on its mechanism. In order to clarify the mechanism of imbibition, a study is carried out and some conclusions are obtained in this paper. Imbibition can be divided into cocurrent imbibition and countercurrent imbibition and the imbibition in reservoirs belongs to cocurrent imbibition. Imbibition can also be classified as horizontal imbibition and vertical imbibition, and vertical imbibition is more complicated than horizontal imbibition. All the imbibitions in reservoirs belong to vertical imbibition. Imbibition also includes one-side-contact imbibition, two-side-contact imbibition and all-side-contact imbibition, and the imbibition in reservoirs belongs to all-side-contact imbibition. Imbibition can also be divided into capillary pressure imbibition and buoyance imbibition. Capillary pressure imbibition can only suck water into rocks and expulse oil, while buoyance imbibition can either suck water into rocks to expulse oil or suck oil into rocks to expulse water. The imbibition in reservoirs belongs to buoyance imbibition. Imbibition in larger pores is faster than that in smaller pores. End-effect may retain some oil in rocks, which will decrease imbibition efficiency. Adding surfactant into water can decrease the influence of end-effect. The larger the matrix block is, the higher the imbibition recovery efficiency will be, but the longer time will be taken for imbibition. The matrix block size and reservoir rock fragmentation degree during volumetric fracturing can be optimized through experiments