Grain boundary sliding during superplastic deformation is generally accommodated by diffusional how or diffusion-controlled dislocation movement in a solid state. However, when stress concentrations are caused around reinforcements in metal matrix composites, a special accommodation process by an accommodation helper such as a liquid phase is required to attain superplasticity. The present paper describes the critical strain rate for accommodation helper mechanism in superplasticity of metal matrix composites, below which sliding is accommodated in a solid state, on the other hand, above which a liquid phase is required for accommodation. The accommodation mechanism maps by using the critical strain rate were roughly in agreement with the experimental results of aluminum and magnesium matrix composites.