It is well known that brittle crack arrest toughness can be improved by grain refinement. The relationship between the fracture surface and the cross-sectional microstructure of the ESSO test pieces by means of EBSD was investigated to clarify the effective grain size of high-strength steel plates. Taking into consideration the fact that a big difference in brittle crack propagation resistance sometimes takes place even if the misorientation between neighboring grains is the same, a new index denoting the propagation resistance referred to crack propagation deviation angle was proposed. The index is defined as an angle between two vectors, which are projected on the ND plane, of the normal vectors of cleavage {100} planes closest to the RD direction of neighboring two grains. It was found that the boundary with the crack propagation deviation angle of 25° or more corresponds to the boundary of the cleavage facet size. The effective grain size evaluated by applying the intercept method on the EBSD data with a high crack propagation deviation angle of the base metal is almost equal to the cleavage facet size. Therefore, the average effective grain size can be estimated without performing any tests to evaluate arrest toughness.