Abstract: The internal friction Q^-1 and the square of vibration frequency f₂(proportional to shear modulus G) were measured as a function of temperature on undoped Pb(Zr_0.7Ti_0.3) O₃ (PZT73) and Pb(Zr_0.3Ti_0.7) O₃ (PZT37) ceramics from 20 ℃ to 500 ℃. Experiments were performed on an inverted torsion pendulum at low frequencies (1~5 Hz). Two internal friction peaks were found in pure rhombohedral PZT73 ceramics. The high temperature peak P_M, which appears at the Curie temperature, originates from the para-ferroelectric phase transition. The low temperature peak P1 is a broadened Debye peak in nature, and it is contributed to the oscillation damping of the domain walls locked by the oxygen vacancies. For the pure tetragonal PZT37 ceramics, another peak P₂,which appears between P₁ and P_M,can be well described by the coupling model. It is thought to be related with the relaxation of oxygen vacancy clusters near the 90° domains.