The complex perovskite Pb(Mg1/3Ta2/3)O-3 is one of the first relaxer ferroelectric compounds that was investigated. The present study reports some new important data of dielectric properties and structure characteristics. A Vogel-Fulcher behavior is observed for the frequency dependence of T-max with a freezing temperature T-f = 144K. Dielectric measurements under high electric field show that the dielectric constant decreases as the field increases, but T,, remains unchanged. The x-ray crystallographic study reveals a cubic structure down to liquid nitrogen temperature. The temperature dependence of the cell parameter is determined and the cluster nucleation temperature is thus estimated to be about 573K. The difference in relaxer behavior between Pb(Mg1/3Ta2/3)O-3 and Pb(Mg1/3Nb2/3)O-3 is discussed.