The accommodation processes for superplastic metal matrix composites are required to release the stress concentration near matrix-reinforcement interfaces because of stress concentration caused by interfacial sliding. An optimum superplastic elongation was found at the temperature where local melting of interfaces was confirmed by in situ transmission electron microscopy observation. It is postulated that superplastic flow in metal matrix composites is controlled by a grain-boundary sliding mechanism accommodated with relaxing the stress concentration by an isolated liquid phase at matrix-reinforcement interfaces.