Abstract: The crack tip opening displacement and crack mouth opening displacement are two indexes evaluating the magnitude of crack opening during fracture. The current investigation presents a study of relationship between the crack tip opening displacement and crack mouth opening displacement throughout the development of cracks using the wedge splitting compact tension fracture tests. It is found that the relationship between them can be characterized by a tri-linear model, and that the two governing points on the tri-linear model approximately correspond to the cracking and critical unstable propagation of cracks, respectively. Furthermore, the influence of depth of specimens on the crack tip opening displacement and crack mouth opening displacement at the cracking moment and unstable propagation moment is discussed. Subsequently, based on a hinge model, a new formula for calculating the crack tip opening displacement is developed. The comparison between experimental values of the crack tip opening displacement and the calculated shows a good agreement.