We aim to understand the effect of plastic deformation on the structure of a polymer glass. For that purpose, we measured by neutron scattering the structure of a polymethylmethacrylate (PMMA) sample stretched both below T-g (T-g - 26 K) and above T-g (T-g + 30 K) over a large length scale range. We observed that the plastic deformation is homogeneous at large length scales and it is affine at scales larger than around the half of the entanglement distance. Moreover at length scales about the monomer size, the structure of the stretched glassy polymer chains remains nearly isotropic but appears also slightly distorted by the plastic deformation. These distortions at the molecular scale are probably the mode of storage of internal energy for stretched glassy polymer.