The rubble mound of composite breakwaters is usually covered with armour stones or armour concrete blocks. These armour materials must have enough weight to withstand heavy wave actions. The Hudson formula with Stability Number Ns is standard one to evaluate the minimum weight of the armour units. Tanimoto et al.proposed a calculation method of the minimum weight using a formula of the Hudson type. However the method is only for normal incident waves and not for oblique waves.
　In the present, report, the minimum weight of armour stones against oblique incident waves is formulated based on the relation between the weight and the water particle velocity on the rubble foundation. The small amplitude wave theory is employed to calculate the water particle velocity for oblique waves. For the breakwater head section, numerical calculations using a kind of mild slope equation are conducted to disclose the velocity distributions around corners of the vertical
wall. A series of model experiments are conducted in a wave basin to investigate the water particle velocity and the damage of the armour stones and the minimum necessary weight of the stone.