The present study investigates the non-isochoric plastic behavior of two thermoplastics, polybutylene terephthalate (PBT) and polyamide 6 (PA6). The uniaxial tensile tests under temperature-controlled conditions are performed and Poisson’s ratio and volumetric strain are evaluated through the measurement of the longitudinal and transverse strains by digital image correlation (DIC). In addition, a mechanical model consisting of a viscoelastic and a non-isochoric plastic model is implemented in finite element software and applied to simulate the mechanical behavior in the uniaxial tensile test. The test results of the both thermoplastics show that the Poisson’s ratio during plastic deformation is less than 0.5. Compared to the test result with the finite element analysis, it is found that the variation of the volumetric strain is in good agreement with the finite element analysis assuming non-isochoric plasticity. These facts mean that the plastic behavior of PBT and PA6 is non-isochoric.