Abstract The deformation mechanism of polyphenylquinoxaline (PPQ-E) film has been studied by theoretical prediction and microscopic observation. The entanglement density, estimated from the plateau modulus on the dynamic mechanical property curves, was used to predict the deformation mechanism of PPQ-E. Polarized light microscope (PLM) was used to observe the morphology of the PPQ-E film during tensile deformation. It is found that the deformation mechanism of PPQ-E is both crazing and shear yielding, which is in agreement with the prediction by entanglement density. The craze morphology of PPQ-E is mainly of fibril craze consisting of micro-fibrils and micro-voids as observed by transmission electron microscope (TEM). The interface between the bulk and craze is distinct. Multiple crazes, blunting of craze tips and homogeneous deformation zones have also been observed.