The tensile stress and the birefringence of polystyrene were measured under elongation at a constant rate up to the elongation ratio of 4 at 115, 105, and 100 degrees C. The tensile stress was separated into two component functions, designated with subscripts R and G, through a modified stress-optical rule, MSOR, considering the effect of finite extensibility of the polymer chain on the stress dependence of the stress-optical coefficient. The R component represents the stress attributed to the polymer segment orientation and the G component to the twist of the polymer chain. At 115 degrees C, the time dependence of the viscosity growth function of the two components, eta(ER)(+) and eta(EG)(+), can be described in the framework of linear viscoelasticity except for a steep increase of eta(ER)(+) at times longer than 1/(2(epsilon) over dot), where (epsilon) over dot is the rate of elongation. It is well-known that this type of steep increase is due to the strong increase of strain measured under elongation at a constant rate. At lower temperatures, eta(EG)(+) decreased with increasing strain rate when the rate exceeds 1/1000 of the characteristic relaxation rate of the G component. The steady value of the elongational viscosity, eta(EG)((epsilon) over dot), at various temperatures supported a master curve when eta(EG)((epsilon) over dot)/eta(EG)(0) is plotted against (epsilon) over dot a(TG), where a(TG) is the shift factor for the G component determined in dynamic viscoelastic measurements. On the other hand, eta(ER)(+) is always close to the linear viscoelasticity except for the steep rise at t > 1/(2(epsilon) over dot). The relaxation rate of the R component was enhanced in proportion to eta(EG)(0)/eta(EG)((epsilon) over dot) when the G component showed thinning. Thus, MSOR analysis simplifies the complicated nonlinear viscoelastic response of amorphous polymers around the glass transition temperature.