In this paper, the effect of environmental conditions, especially humidity on the fatigue strength of polymers is discussed. Acrylonitrile butadiene styrene (ABS) and acetal homopolymer (POM)which are seen in industrial applications were used. The observation of fracture surface by SEM and quantitative analysis of the fatigue specimen by scanning acoustic microscope (SAM) were carried out to make clear the influence of environmental conditions. As a result, it was found that ( 1 ) ABS and P0M in high-humidity conditions exhibit a shorter fatigue life than in low-and medium-humidity conditions, and ( 2 ) the fatigue strength of polymer is considered to be influenced by elastic properties of the surface of the specimen. With the measurement of LSSCW velocity, elastic properties at microareas of the polymer became clear.