Abstract: Cavitation is the strongly compressible flows, where the cavitation compressibility effects could be closely associated with the cavity instabilities and dynamics. To investigate the cavity structure evolution and dynamics under both the re-entrant jet and shock wave mechanisms in compressible cloud cavitating flows, experiments were conducted in the divergent section with 10° in a venturi, using the simultaneous sampling technique to synchronize the observations of transient cavity behaviors with the wall-pressure signals measurements in cloud cavitating flows. A novel image processing algorithm is developed to analyze the transient cavity structures in detail. Based on the developed image processing method, the attached-type sheet cavity and shedding cloud-like cavity structures can be studied quantitatively. Results showed that unsteady cavity behaviors under re-entrant jet mechanism (RJM) can be divided into three stages: 1) the attached cavity growth, 2) the re-entrant jet formation and movement, and 3) the attached cavity breakup and cavity cloud shedding, collapse. The transient cavity structures under shock wave mechanism (SWM) present periodic behaviors: 1) the attached cavity growing, 2) the shock wave generating and propagating, and 3) the attached cavity collapsing and shedding. The period of re-entrant jet development is larger than that during the shock wave propagation. The interactions between shock wave and cavity cause significant local void fraction reduction, which induces complex cavity dynamics. During the shock wave propagation, both large pressure fluctuations and pressure peaks are captured, while in the process of the re-entrant jet movement, relatively smooth pressure fluctuations are measured, with small pressure fluctuations increase at the re-entrant jet head. Furthermore, the behaviors of attached-type sheet cavity, and shedding cloud-like cavity are totally different under re-entrant jet and shock wave mechanisms,indicating different mass transfer processes in different cavity shedding mechanisms.