The low-frequency internal friction (0.1 Hz < f < 10 Hz) of (100-x%)Pb(Mg1/3Nb2/3)O-3-x%PbTiO3 (x=0, 13, 23, 33) ceramics has been measured in the temperature range between 90 and 500 K. All of the internal friction peaks show a height that is inversely proportional to the measurement frequency, thus they can be attributed to first-order phase transitions. The phase transitions between rhombohedral, monoclinic, and tetragonal phases in ceramics with x=33 occur over a very broad temperature range from 150 to 400 K, indicating that the giant piezoelectric effect in this material may be attributed to a field-induced phase transition process.