The minimum particle size for cavity nucleation was experimentally investigated for superplastic 2124Al and 6061Al composites reinforced with 20vol% Si3N4 particles. From the minimum particle size for cavity nucleation, the critical stress at the interface for cavity nucleation was estimated to be about 5 MPa. The local stresses at the interface for the particles of average size where large elongation above 200% is attained in a temperature range below the partial melting temperature are lower than the critical stress. It may be therefore suggested that large elongation is attained without the accommodation helper process of liquid under the conditions that the local stress at the interface is lower than the critical stress at the interface for cavity nucleation.