The yielding and plastic behaviour of atactic poly(oxypropylene) (a-POP) was simulated under the uniaxial compression condition using atomistic modelling, and compared with the result under the uniaxial extension condition. Typical stress-strain curves showing yielding and plastic behaviour were obtained under the uniaxial compression condition. It was observed that the stress under uniaxial compression is greater than that under uniaxial extension. This observation suggests that the difference of stress level between the two conditions is not related intrinsically to the structural defects in the sample. When the energetic state of a-POP was analysed by monitoring the change of energy component with strain, it revealed that the change of total energy with strain was governed dominantly by non-bonded interaction, such as van der Waals and Coulombic interaction. The role of Coulombic interaction is more important under compression than under extension.