The local compression (LC) technique is used to achieve a straight crack front in weld specimens. However, some interested parties have reported that it results in an excessive underestimation of the fracture toughness value. In a previous study, the authors investigated an improved LC condition for securing a straight crack front while maintaining the fracture toughness value through a numerical analysis and fracture toughness tests. As a result, it was found that two local compression conditions which do not apply compression at the future notch tip are superior to the typical LC. However, since these proposed conditions were identified based on the experience of the authors, there may be others that will yield better results. In this study, Bayesian optimization was conducted to identify an improved LC condition. Because the results indicated that the compression area and position are important for improving LC, the conditions for a validation experiment were determined based on these parameters. The proposed LC condition achieved a straight crack front, and the fracture toughness was higher than under the condition recommended in ISO 15653. Therefore, the authors concluded that the condition for improvement of LC was successfully identified by the Bayesian optimization.