As one of optical methods, the method of caustics is useful for determining the stress intensity factor. In this paper, the method of caustics was applied to the specimens with an oblique crack. The results obtained are summarized as follows: (1) The method of caustics is a useful technique to determine the stress intensity factors KI and KII separately under mixed-mode loading. (2) The existing theory used for measurement of this method is effective only when the initial curve radius r0 is larger than the minimum initial curve radius r0min determined in this study. It is recognized that the value of r0min decreases as the ratio of KII to KI increases. (3) The 3-D stress field exists in the vicinity of crack tip, while the state of stress is nearly planestrain deformation in the case of mode I loading. In the case of mixed-mode loading, the state of stress is approximately plane-stress deformation as the ratio of KII to KI increases. (4) A correction is proposed to obtain the values of K from caustic images in the case of r0<r0min.
