Seawall designs must consider the type of armour unit, wave height, and economic factors. Selecting the appropriate size and type of protective unit can prevent excessive damage to structures at sea. In this study, the stability of the A-Jack armour unit was analyzed through physical model testing. Among the three structural slope variations in this test, AJack had the highest stability at a slope of 1:2. At a wave height of 0.1 m, the A-Jack was still not damaged, but the Tetrapod suffered damage of 0.43% and the BPPT-Lock of 0.83%. Antifer type 2 has a damage percentage of 2.3% with a stability number of 2.23, whereas A-Jack causes damage of 5.35% with a stability number of 2.26. As the stability coefficient decreases with increasing structural steepness, A-Jack has better stability than the Antifer.