This paper describes the complicated flow associated with the three-dimensional surface-mounted square cavities. The experiment was carried out in two types of wind tunnels and a water channel. The effect of the aspect ratio (cavity length to depth : L/D=1-8) of the cavity has been investigated in terms of the overall cavity drag increment, the surface pressure distribution and the velocity profiles in the flow inside the cavity. The flow pattern inside the cavities was observed by the flow visualization. The three-dimensionality of the primary vortex in the cavities have been estimated from location of the centre along the span of the cavity. Consequently it was found that (i) a remarkable decrease of the drag occurs at L/D=4, and (ii) two cells structure for L/D=4 and three cells structure for L/D=8 exist in the velocity profile inside the cavity.
