Atomic force acoustic microscopy (AFAM) has been developed in order to evaluate mechanical properties of the materials at nano-scale. The SiO_x films on the silicon wafer and glass slide were prepared by plasma enhanced chemical vapor deposition (PECVD) and their properties were characterized by atomic force acoustic microscopy (AFAM). The images of the amplitude of the vibrating cantilever were visualized for the sample vibrating at the ultrasonic frequency and the characteristics of the images were also discussed at the different excitation frequencies. The results showed that the acoustic amplitude images can provide information about the local elasticity and the subsurface defects of the materials qualitatively.