The deformation and strength of rocks are significantly influenced by the crack closure behavior. In this study, the crack closure effect and its quantitative characterization are studied. First, based on the widely used effective medium theory, the axial strain is divided into the matrix axial strain and the crack axial strain. Three sets of test data are then used to establish a negative exponential model for the relationship between the crack axial strain and the axial stress. It is found that the crack closure behavior is well captured by the proposed model. The crack closure strain can be used to some extent to represent the microcrack quantity (or microcrack density) in the rock sample. The variation of the microcrack density under uniaxial and triaxial compressions is well described by using the crack closure strain.