Dielectric behavior and ferroelectric hysteresis loop of(1-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3(PMN-PT)(x=0.2)ceramics have been investigated under different ac electric fields from 12.5V/cm to 4kV/cm above the room temperature.As the temperatute increase,there are two peaks appear in the dielectric constant under a low AC field above the room temperature,which one is Tm and the other one is R – T phase transiton(or macro-domain to nano-domain transition),but only one peak apper in the dielectric constant under a high AC field,which peak temperature is lower than Tm but close the the R-T phase transition.