The longitudinal Brillouin back-scattering spectra, dielectric permittivities and polarization-electric field (P-E) hysteresis loops along the [111] direction have been measured as a function of temperature for a relaxor ferroelectric single crystal(PbMg1/3Nb2/3O3)0.67(PbTiO3)0.33(PMN-33&percent;PT). A sharp ferroelectric phase transition, which is associated with a minimum in phonon frequency and an abrupt change in polarization, was observed near 425 K. As temperature decreases, a diffuse phase transition which is associated with a broad frequency-dependent dielectric spectrum and a gradual evolution in polarization, was detected near 350 K. In addition, the nature of the thermal hysteresis for the dielectric permittivity confirms that these transitions (near 350 and 425 K) are diffuse first-order and first-order, respectively. The field-cooled-zero-field-heated dielectric permittivities show that the dc bias field induces a long-range ferroelectric phase transition at 357 K and imply the phase coexistence of a short-range electric ordering and a long-range electric ordering in the region of ∼330–380 K. ©2000 American Institute of Physics.