The tensile property of PC/ABS was investigated with an optical image analysis method in the paper. With the consideration of polymer micro mechanism, a three-stage phenomenon based model is used to describe the true stress-strain curves. The regression constants obtained with the method could be described with definite physical meanings. It is shown that two lateral contractions are not identical at the later necking stages. The fracture strain could be used as a criterion to describe uniaxial tensile fracture.