It is widely believed that grain boundary sliding, accommodated by diffusion, which leads to a Newtonian viscous behavior (σ ∝ ˙ε) and grain matrix deformation exhibiting a power law creep behavior (α ∝ ˙ε^m), are the two concurrent deformation phenomena occurring at elevated temperatures in metals, particularly during superplastic flow. A deformation model for this combined behavior has been developed in which the grain size and its distribution are taken into account. Strain compatibility relations have been utilized to predict material flow behavior under loading, load relaxation, and steady-state conditions. Comparison with experimental results shows that the sigmoidal σ-˙ε relationship for superplastic metals is explained well by the model. The model also simulates other experimental aspects as well as internal stress development during elevated temperature flow.