Tensile flow of polystyrene and polycabonate in the glass transition region was examined. The behavior at the lower yield point was compared with that at the upper yield point, where the latter is the point of the maximum stress of tensile stress-strain curve right after the elastic limit, and the former is the point of the minimum stress beyond the upper yield point. The upper yield point had been considered as the point where purely plastic flow begins. It was found that the upper yield point was influenced significantly by the glass transition, while the lower yield point was not. The yield behavior at the upper and lower yield points were examined in terms of a phenomenological theory by Brady et al. and molecular theories by Robertson and by Argon for plastic flow of glassy polymers. No satisfactory agreements of the theories with our results for the upper yield point were seen. However, Robertson's structural-temperature theory showed a fairly good agreement with our results obtained at the lower yield point. The results obtained on polystyrene and Polycarbonate coincided qualitatively with those of poly (methyl methacrylate) reported previously.