The mechanical behavior of poly(trimethylene terephthalate) (PTT) in an amorphous state was simulated under uniaxial extension-compression conditions through atomistic modeling. The elastic properties obtained from the well-defined structure were comparable with those obtained from experiments. Typical stress-strain curves showing yielding and plastic behaviors were obtained under both extension and compression conditions. It was observed that the stress under uniaxial compression is higher than the stress under uniaxial extension with the same strain. The absence of structural defects in samples for simulation leads us to conclude that the difference of stress levels between two conditions is not intrinsically related to structural defects. Various analyses of chain conformation such as bond orientation, the dihedral angle, and Voronoi volume tessellation shows that the PTT chain under extension undergoes conformational changes different from those under the compression condition. When the change in the energetic state of PTT is calculated as a function of strain, it is revealed that the dihedral angle torsion energy and the van der Waals interaction energy play more important roles in deformation under extension than under compression.