The caustics method is simulated by three-dimensional finite element analysis. CT specimens with different thicknesses and different crack front shapes are analyzed and the crack tip fields are compared with HRR solutions. Caustics are also drawn using the displacement field obtained by the FEM analysis. It is shown that the caustics method can estimate the J-Integral value with good accuracy when the initial curve is located in the limited region where the stress fields agree with the HRR solutions. The diameter of the caustics is measured experimentally by using the same CT sepcimens, and it is shown that the experimental results agree with those of the numerical simulation well. It is concluded that numerical analysis is necessary to estimate the location of initial curves to accurately measure the elastic-plastic fracture parameter J.