The development of crazes in thin films of glassy polymers is a result of a micronecking process. Hence, when sandwiched between ductile poly(phenylene oxide) (PPO) films, a brittle glassy polystyrene (PS) film can be plastically deformed to very large strains without crazing or cracking. However, the stability of this superplastic behavior is strongly dependent upon thickness of the outer ductile films. A sharp ductile-brittle transition in the sandwich thin film structure was observed as the PPO thickness decreased. A simple mechanical model built upon the competition between the necking force, associated with orating,, and the constraining force, due to the ductile films, was utilized to analyze the stability of this superplasticity. The result of the mechanical analysis is in good agreement with the experimental data. The mechanical calculation indicates that the constraining force from the outer layers should be at least 60 MPa greater than the necking force to ensure the superplasticity. The defect size corresponding to this excess constraining stress is around 18 nm, comparable in size of the correlation length of polystyrene chains.