The dielectric properties and phase transformation of poled <001>-oriented Pb(Mg1/3Nb2/3)O3-x%PbTiO3(PMNx% PT) single crystals with compositions of x = 20, 30, and 35 mole% are investigated for orientations both parallel and perpendicular to the [001] poling direction. An electric-field-induced monoclinic phase was observed for the initial poled PMN- 30PT and PMN-35PT samples by means of high-resolution synchrotron x-ray diffraction. The monoclinic phase appears from .25oC to 100oC and from .25oC to 80oC for the PMN-30PT and PMN-35PT samples, respectively. The dielectric constant (ε)- temperature (T) characteristics above the Curie temperature were found to be described by the equation(1/ε . 1/εm)1/n = (T . Tm)/ C, where εm is the maximum dielectric constant and Tm is the temperature giving εm, and n and C are constants that change with the composition. The value of n was found to be 1.82 and 1.38 for 20PT and 35PT, respectively. The results of mesh scans and the temperature-dependence of the dielectric constant demonstrate that the initial monoclinic phase changes to a single domain tetragonal phase and a to paraelectric cubic phase. In the ferroelectric tetragonal phase with a single domain state, the dielectric constant measured perpendicular to the poling direction was dramatically higher than that measured in the parallel direction. A large dielectric constant implies easier polarization rotation away from the polar axis. This enhancement is believed to be related to dielectric softening close to the morphotropic phase boundary.