Abstract: Though dilation of shales plays an important role in petroleum engineering, the research of the dilatancy of shales is currently rare. As the confining pressures and the initial porosities of rocks control the dilatancy, a conceptual partition of different dilation zones is proposed. In order to obtain the critical confining pressure of shales at given porosity, a group of shale specimens are subjected to compression tests in the lab. Combining with the dilatancy index, a power function is used to fit the relationship between dilatancy indices calculated by experimental results and confining pressures. The dilatancy indices of shales are found to be decreased with increasing confining pressures as expected. Then a critical confining pressure beyond which the shale specimens would stop dilation is predicted by the power function. And finally some different opinions about the dilation process of rocks are also discussed. So as to get the critical curves in confining pressure verse porosity chart which is crucial in dilation prediction of shales, some more shale specimens should be carried to compression tests in the future. Introduction Dilatancy may be described as the change in volume associated with shear distortion of an element in the material (Vermeer and Borst, 1984). The dilation angle was first introduced to characterize the dilatant property of a granular material by Bent Hansen in 1958. The dilatancy affects the loading-carrying capacity and the spread of the plastic zones (Cox et al., 1961; Zaadnoordijk, 1983; Vermeer and Borst. 1984). Dilatancy is a crucial factor in the stability design at shallow depths, where confinement pressure is low (Reynolds, 1985). The importance of dilatancy on earthquakes is also studied (Frank, 1965; Scholz, 1968; Zoback et al., 1975; Nur, 1975). The dilatancy influences the hydraulic behavior in rocks (Zoback and Byerlee, 1975; Zhang et al., 1994; Peach and Spiers, 1996; Zhu and Wong, 1999; Heiland and Raab, 2001; Simpson et al., 2001; Tang et al., 2002) which may be used in hydraulic fracturing in petroleum industry. Islam mentions the potential applications of dilatancy in numerical modeling of borehole stability (Islam and Skalle, 2010), sand production analysis, reservoir compaction, stress arching, network fracturing, and underground storage of CO 2 (Islam et al., 2013).