SYNOPSISThis paper deals with the stability of vertical walls in mixed type breakwaters. A new concept named “probable sliding distance” is presented to evaluate the stability of caisson walls under the action of irregular waves.The fundamental idea of probable sliding distance lies in the examination of the behaviour of structures beyond the stability limit for evaluating their ultimate safety. The frequencies and the amounts of the displacement of model caissons mounted on rubble mounds were observed in an irregular wave basin. The intensity of wave pressure and the coefficient for calculating the sliding distance have been obtained from these experiments.The “probable sliding distance” is defined as an expected total displacement of a caisson due to irregular waves of a certain magnitude and duration. The conventional design standard is examined on the basis of this idea and a revision of stability calculation is proposed.