Two types of samples were prepared from the same epoxy precursor; one was cured with 4,4'-DDM to form a cross-linked molecular structure and the other was polymerized by aniline to form a linear molecular structure. After it was shown that the modified stress-optical rule is valid for the birefringence data of the samples, birefringence change was monitored during uniaxial elongation in the glassy state as well as in the rubbery state. The results were analyzed within the context of modified stress-optical rule to investigate the plastic deformation of epoxy glass. For the linear sample, rubbery stress increased after a yield point, while glassy stress remained almost constant. On the other hand, for the cross-linked sample, rubbery stress increased cooperatively with glassy stress from the beginning of elongation, and such cooperative increase in stresses continued after the strain passing an upper yield point. (c) 2005 The Society of Rheology.