Abstract: During crack initiation and propagation, the deformation fields around a crack tip play an important role. In essence, the deformation at the tip of any crack is three dimensional. The investigation of the three dimensional deformation will be useful to better understand the fracture behavior of the material. In this paper, three dimensional deformation fields around a crack tip are discussed theoretically, and series expansion solutions are derived. Comparison of theoretical solutions against series expansion solutions is made. The results indicate that the series expansion solutions converge very fast, taking the third item, it is very close to the theoretical solutions. Three dimensional deformation fields are obtained through experiments. Furthermore, the in plane deformation fields and out of surface deformation fields are compared with theoretical solutions and numerical analysis results respectively. It is shown that the deformation fields around a crack tip always display three dimensional features, and the deformation in any direction can not be omitted at mesoscopic level.