The rubble mound of composite breakwaters is usually covered with armor stones or armor concrete blocks. However the damage of armor units is almost appeared at the breakwater head. The minimum weights of the armor units at the breakwater head is 1.5 times the value of the breakwater trunk. This paper aims to elucidate the damage mechanism of armor units at the breakwater head of a composite breakwater by numerical simulation with a 2 or 3-dimensional numerical wave tank `CADMAS-SURF' under regular wave action. Numerical results are compared with experimental data. At the last, the numerical wave tank is applied to real disaster of the armor units of the composite breakwater.