The mechanical consideration of the bimaterial interface is a very important problem on the evaluation of the material. Then, the stress intensity factor K for a crack along the bimaterial interface has been measured by the method of caustics. The effect of initial curve for the measurement accuracy is examined. As a result, it is possible to apply this method to the bimaterial jointed materials when the maximum diameter D of the image is used and the value r_D of initial curve is larger than the value r_D^<min> obtained in this study. And, the value r_D^<min> is about half a value of the specimen thickness when the same materials are jointed, it becomes smaller when the different materials are jointed. There is a close correlation between the size of 3-D stress field and Young's modulus ratio of each specimen.
