Thermoplastics are viscoelastic-viscoplastic materials whose mechanical behavior depends on temperature and strain rate. In this study, the viscoelastic-viscoplastic characteristics of polybutylene terephthalate are evaluated on the basis of the mechanical model consisting of Schapery nonlinear viscoelasticity and Perzyna viscoplasticity. Also, the viscoelastic-viscoplastic behavior of the polybutylene terephthalate is predicted by finite element simulation. The evaluation reveals that the initial yield stress depends on temperature and strain rate; however, viscosity depends on only strain rate. Furthermore, the rate sensitivity is independent of temperature and strain rate. The finite element simulation using identified viscoelastic-viscoplastic characteristics can reproduce the stress–strain relationship in the uniaxial tensile tests under various testing conditions. It is expected that the time-temperature superposition principle can be applied to not only the maximum stress but also the initial yield stress.