Nonlinear viscoelastic behavior of glassy poly(methyl methacrylate) (PMMA) subjected to high tensile creep stresses was studied. Right after the loading of creep stress, a rapid decrease of strain rate as a function of strain was observed, indicating behavior of linear viscoelastic solid. Then after showing a sharp minimum, the strain rate gradually increased due to nonlinear flow and reached to almost constant maximum values at steady flow state, and then slightly decreased. Strain range in which the nonlinear flow was observed did not strongly depend on the magnitude of creep stress and temperature, although magnitude of strain rate was obviously affected by these conditions. Nonlinear single relaxation time τ for the glass in the nonlinear flow range of strain was evaluated by means of a simple mechanical model. The nonlinear relaxation time τ decreased with increasing time t to a steady value appeared in the steady flow state. A quantity τ/t was found to be an almost unique decreasing function of strain independently of differences in magnitude of creep stress and temperature. This characteristic dependence of relaxation time τ on strain was considered as a result of a constant decreasing slope of τ-t relation, which slightly depends on the magnitude of stress, during transient flow state under constant high stresses.