Cavitation was quantitatively investigated at 8 MPa and 833 K for a high-strain-rate superplastic Si3N4p/Al-Mg-Si composite, where the testing temperature of 833 K was slightly above the onset temperature for partial melting. Most of the cavities were very small (≤1 μm), and cavity development was not significant. Based on a theoretical analysis, when stress concentrations around reinforcements are relaxed by the presence of a liquid phase, excessive cavity development is suggested to be limited. This view is in agreement with the experimental results.