Abstract <p>This paper presents our results on the creep of polymethyl methacrylate (PMMA) under cyclic load. We have analyzed the viscoelastic behavior of PMMA under normal operating conditions, before damage to the material. Creep during continuous deformation is a combination of two processes: creep acceleration due to a rise in stress and creep deceleration over time. In the case of an instantaneous rise in load, only the former process is operative; during holding under constant load, only the latter. For both processes, we have derived equations of viscoelastic state which relate the viscous strain acceleration to the elastic and viscous strain rates and the current elastic strain level. The equations are applicable in the stress range from the creep limit to the forced elasticity limit and in the case of recovery after complete unloading. Since the equations do not include time or accumulated viscous strain in explicit form, they can be used in the case of a process with an arbitrary law of the rise in strain or stress. Using results obtained in cyclic tests at various strain rates in the loading and unloading stages, we have derived equations of state for various combinations of the magnitude and sign of the elastic and viscous strain rates. The equations have been used to model viscous deformation of PMMA according to a preset law of the variation in elastic strain. Comparison with experimental data has confirmed high modeling accuracy. We have identified a number of successive stages of viscoelastic deformation in relation to the stress level: elastic (at stresses below the creep limit), instantaneously viscous, viscous, and viscoplastic stages. The viscous strain does not rise in the first stage, rises only with increasing load in the second stage, and rises during holding as well in the third stage. The viscoplastic stage corresponds to accumulation of irreversible strain, which persists after recovery.</p>