In the design of harbour structures, the determination of structure scale or the area of soft ground to be improved is dependent on the safety factor of stability calculated by the circular arc slip method. In order to take into account of the inhomogeneity of real ground, some reliability analyses are carried out by use of Monte Carlo simulation. The construction cost, probability of failure and re-construction cost after structure failure are calculated, then the optimum safety factor is found out in such a way that the expected total cost of construction and re-construction after failure is at its minimum value. Main results of this study can be summarized as follows:
a)　The safety factor for stability analyses of harbour structures by the circular arc slip method must be chosen based on the importance of structure, ground inhomogeneity, reliability of shear strength data and the construction condition. For the examples of gravity and sheet pile types of revetments, theoptimum safety factor may empirically be given as below:
　Fopt=1.05+0.85(1+log10n)V
In this relation, n is the lost cost ratio and is defined by n = re-construction cost/ construction cost. The value of n may vary within a range of 0.5～10. Whille V is variance coefficient which is used to express the variant degree or distribution property of shear strength of the construction ground.It may vary within a range of 0.1～0.2.
b)　As concerning the lost cost ratio n and the variance coefficient V, the authors suggest that safety factor be selected according to the importance of the harbour structure as follows:
　For the case that the ground is relatively homogenous and obtained shear strength data are reliable;
　Not very important harbour structure(n<1) 1.15
　Usual harbour structure(1<n<3) 1.20
　Very important harbour structure(n>3) 1.25
　For the case that the ground in inhomogeneous or obtained shear strength data are not so reliable;
　Not very important harbour structure(n<1) 1.20
　Usual harbour structure(1<n<3) 1.25
　Very important harbour structure(n>3) 1.30
　It should be noted that the importance of a structure is evaluated by the lost cast ratio n, so it seems that more detail studies on the valus of the lost cost ratio n are required to be done as related with social and economic impacts when a structure failed.
c)　If a breakwater is designed according to the current technical standard which requires that stability safety factor by Bishop's arc method be equal to or greater then 1, there exists a probability of failure about 50～60% when a designed storm or high wave occurs. However, if considering from a viewpoint of the probable natures of wave force and bearing capacity of mound beneath a caisson structure, it is found that the expected settlement due to mound failure will be less than 10cm for triangle shape of wave force, and less than 50cm for sine shape of wave force during an endurable life of 50 years.