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We discover a new cavitation phenomenon in amorphous polymers sandwiched between two thick slide glasses from the static melt induced by thermal treatment. By quenching atactic polystyrene (aPS) samples from the static melt under the glass transition temperature (Tg) and annealing them above Tgfor thick slide glasses, cavities are created to relax the negative pressure. Cavity growth undergoes an Ostwald ripening-like process in cases of large molecular weight, while it undergoes a viscous fingering process in cases of small molecular weight. The induction time for cavity formation is found to decrease with the increase of the annealing temperature and with the decrease of molecular weight. In contrast, no cavities are observed in an aPS sample between two thin cover glasses because the negative pressure can be relaxed by bending the whole thin cover glasses.