In the previous reports, a simple fracture mechanics concept have been successfully applied to the analysis on the experimental results of several kinds of brittle crack arresters using medium size specimen (500 mm wide) by one of the authors. The length of arrested crack considered was limited within 100 to 350 mm. In this report a special reference is made to the propagation-arrest of a extremely long crack. The experimental results using wide specimen (1, 000 to 2, 400 mm wide) showed the simple extrapolation of the concept valid for shorter crack could not be applied to the case of arrest of a extremely long crack. As a practical solution to this problem, some consideration on the saturating characteristics of nominal K value of a propagating long crack have been made by the authors. A new concept of “effective K value” and some fundamental aspects of design philosophy for brittle crack arresters are proposed on the basis of those considerations.