Yielding of a model glassy polycarbonate under tension was studied using the molecular mechanics method. The model structures were stretched by changing the cell dimensions consistent with the applied strain and the assumed Poisson ratio. After the deformation, the model structures were optimized. The resultant stress-strain curves clearly indicate the yielding of the model polymer near 13% strain. The nonbonded van der Waals interactions represent the dominant contribution to the overall strain energy. The yield stress and yield strain obtained from the simulation compare favorably with the experimental data if the latter are extrapolated to 0 K. To further examine the yielding phenomenon, the same deformation scheme and an affine model were applied to a model atomic glass. The results support the idea that the nature of yielding is the inflection exhibited on the Lennard-Jones potential representing the van der Waals interactions. The structural changes of the model polymer were studied through analyses of the Voronoi volume distribution of atoms, the segmental orientation, and the overall chain deformation and orientation. Voids were developed during the deformation.