Abstract: In this paper, ultrasonic fractography method is used to test several types of PMMA specimens with one-side-through crack. The ultrasonic lines in the fracture surface of these specimens are obtained. Three-dimensional J integrals of the specimens are evaluated based on the finite element code of ANSYS. There are similarities between three-dimensional J integrals evaluated under different crack lengths and ultrasonic lines in the specimens' fracture surfaces in shape and protruding direction. Three-dimensional J integrals tend to be constant after line crack fronts are changed to certain predicted curve shapes, which verifies the hypothesis that the magnitudes of J integral vectors are equal on every pointof propagating crack fronts.