Atomic force acoustic microscopy (AFAM) has been developed in order to evaluate the mechanical properties of the material at the nanometer scale. The SiOx film on the silicon wafer was prepared successfully by plasma enhanced chemical vapor deposition (PECVD). In order to characterize the elastic property of the SiOx film , the images of cantilever amplitude were visualized when the sample was excited at 100kHz and 400kHz frequency. The acoustic amplitude images also were discussed at the different exciting amplitudes. The results showed that the acoustic amplitude images can provide the information about local elasticity of the materials