In this study, waves with the heights higher than H-1/3 in an irregular wave train are called as extreme waves and defined with the help of extreme wave parameter, alpha(extreme). In order to see the effect of extreme waves on the design weight of armour stone, stability analysis is carried out based on the hydraulic model test results. The test results of high alpha(extreme) cases (HE) and low alpha(extreme) cases (LE) are compared with currently used van der Meer's formulae with permeability factor P = 0.4 and 0.45 and Hudson formula by using H-1/3 and H-1/10 in terms of the design weight of armour stone. As a result of the comparison, it is found that Hudson formula by using H-1/3 underestimates the necessary armour weight. Usage of H-1/10 instead of H-1/3 in Hudson formula doubles the weight which seems overestimated when Irribaren number is away from the transition zone in which both wave run-up and run-down forces become effective. However, it seems underestimated near the transition zone where experiment case HE gives higher armour weights. When the design weight of armour stone is calculated by van der Meer's formulae with P = 0.4, it may be necessary to increase the weight up to 30% in the case of high extreme waves. On the other hand, van der Meer's formulae may overestimate the weight 14% when the extreme waves are low. (c) 2007 Elsevier Ltd. All rights reserved.