An experimental investigation is conducted on supersonic internal flows over a rectangular cavity at mach number M=1.83. In this paper, the basic characteristics of the cavity flow fields and the effect of cavity dimensions (the length to depth ratio) are discussed. Time-averaged and unsteady pressures are measured and the flows are visualized by the schlieren and the oil flow techniques. The basic characteristics of the supersonic cavity flow, namely three-dimension-ality and the shock/shear layer system, are recognized. A longer cavity has a large circulation flow and a pair of small three-dimensional vortices due to the separation on the side walls. A structure of the flow is highly three-dimensional. The shorter cavity has only one vortex and the flow is essentially two-dimensional. The unsteady pressure data reveals the dominant frequencies of the cavity flow phenomena. In this investigation, the features of the basic cavity flow fields are clarified.