In this paper, the micromechanism of the deformation process during fatigue testing was discussed. ABS and polyacetal, which are applied in industrial service using were used. Quantitative analysis of the damaged area on the tensile and fatigue specimen by scanning acoustic microscope (SAM) was performed. Furthermore, TEM observation of the fracture surface was carried out to clarify the micromechasism of the damaged area. As a result, it was found that ( 1 ) in tensile testing, the damaged area of the specimen was widely dispersed. On the other hand, in fatigue testing, the damaged area was concentrated near the fracture surface: ( 2 ) the micromechanism of the deformation process could be explained by the nucleation and coalescence of crazes or microcracks.