Post‐yield plateau of a true‐stress versus strain relation for glassy polymethyl methacrylate (PMMA) is illustrated to be a state of steady flow. A novel rate analysis applied to this steady flow provided a unique functional relation between the activation energy ΔHand the activation entropy ΔS. This functional relation for the PMMA glass is demonstrated to agree with a function of ΔHvs ΔSobtained from the relaxation data for the melt of the same PMMA sample. This agreement was also confirmed for polyvinyl chloride, polycarbonate, and polystyrene, thus giving direct evidence verifying a change of the metastable glassy structure into a liquid‐like structure in the polymer glasses. Hence, the yielding in the polymer glasses is convincingly concluded to be a transition phenomenon due to this structural change.