Abstract: In tapping mode of Scanning Probe Acoustic Microscope (SPAM),based on the characteristics of probe vibrated periodically as well as both driving and damping forces of free vibration are small in the contact time,the impact motion equation between probe and sample is solved.The maximum indentation depth and impact time can be found,and the relations between them and reduced Youth modulus,interface adhesion energy as well as probe radius are obtained.It is shown that the phase contrast in tapping mode is produced by local properties of the sample,probe amplitude and setpoint,as well as topography of the sample.And the contrast on the phase images of nanodiamond is predicted as 72.59° and it is in good agreement with the measured average result 73.2°±8.2°.The contrast on phase images of optical films is explained reasonably.It shows that Phase imaging in SPAM tapping mode can detect the surface properties of the sample with nanoscale resolution.