We present a theory to account for how a stress field can induce plastic flow in glassy polymers. We consider a molecular model in which chains are constituted from isotropic oriented elementary links embedded in a deforming continuum. The shear-stress field causes a redistribution of such links governed by a balance equation in orientation space. After detail calculations of the number of oriented units in certain directions, the net jump rate of the molecular links is given by an exponential form, according to the transition state theories of Adam and Gibbs [6]. These considerations are compared with the study of Boyce et al. [5] that included the effects of deformation rate, pressure and strain softening. The calculation of the plastic properties for polymethyl methacrylate and polycarbonate for various types of deformation has been made, and the corresponding results fitted the experimental data reasonably well.