On the basis of the frequency-sweep tapping mode atomic force microscopy (TMAFM) experiments for polystyrene (PS) and poly(dimethylsiloxane) (PDMS), the validity of the harmonic approximation for describing the phase shift of TMAFM was tested by evaluating the analytical expressions derived from the approximation. Our study shows that the phase shift is almost quantitatively described by the harmonic approximation. By analyzing the results of these experiments, it was also examined how the viscoelastic properties of a polymer affect the nature of the tip-sample interaction. The nature of the tip-sample interaction is strongly affected by the sharpness of the tip and by the elasticity, viscosity, and plasticity of a polymer sample. On a strongly glassy, brittle sample with a low impact strength, such as PS, the phase shift can be dominated by the attractive force with the use of a dull tip, while on a strongly elastic sample, such as PDMS, the phase shift, can be dominated by the indentation force with the use of a sharp tip.