Today, steel plates of container carriers become thicker because larger ships are needed for carrying more baggage and reducing the transportation cost. Thus, even though brittle fracture occurs, arresting the crack is essential as “double integrity” for structures. It is directly effective to improve the arrest toughness, which is the material resistance against the brittle crack propagation. However, there was not established theory to explain the dependence of the microstructure on arrest toughness. In particular, although it is empirically known that of the finer grain size makes higher crack arrest toughness, the quantitative relationship was not clarified.Recently, Shibanuma et al. proposed a multiscale model to simulate the brittle crack propagation/arrest behaviour qualitatively. However, the microscopic energy absorbing mechanism in the model is too simple to reveal quantitative prediction of arrest toughness. According to the above background, we conduct an experiment to quantitative clarify the relationship between arrest toughness and grain size, and then modify the above multiscale model based on the experimental results. Consequently, the results obviously shows the relationship between arrest toughness and grain size of steels. The experimental result was contrary to the result of microscopic model. Thus, it is necessary to correct a numerical expression of the microscopic energy absorbing mechanism in the model to accurately simulate the actual brittle crack propagation and arrest behavior in steel of arbitrary grain size.