In the present paper, the authors investigate many examples of two material constants, C and m , which are involved in the Paris Formula, da / dN = C (Δ K ) ^m . Some correlation exists between C and m , irrespective of variety in mechanical properties, metallographic structures, mean stress and compliance of loading system. Crystallographic features or other structure-insensitive characteristics may affect the correlation, but a definitely close one is considered available for the present proposal of design basis, as far as the conventional steels such as the hull-structural are concerned. Based on the C - m correlation, the authors demonstrate some formulation for evaluating the safety of hull structures. Loading spectrum, initial and final sizes of defect and threshold stress intensity factor are also taken into account.