Temperature- and electric (E)-field-dependent domain structures and phase transformations in a (001)-cut Pb(Mg1/3Nb2/3)0.6Ti0.4O3 (PMNT40%) single crystal have been investigated by polarizing microscopy and dielectric permittivity. The unpoled crystal has tetragonal phase domains (15–60 μm wide) with polarizations along  and , which are separated by 90° domain walls. They coexist with a small fraction of the monoclinic phase domains near 200 K. The tetragonal domains increase rapidly at the expense of the monoclinic domains as the temperature increases toward room temperature. This may account for the steplike dielectric anomaly in ε′ from around 280 K. The whole crystal reaches the cubic phase near 464 K. The E-field-dependent domain structures at room temperature show a significant change near 11 kV/cm when the polarizations rotate to the tetragonal  (T001) direction through the monoclinic phase. Microcrackings begin to develop near 12.5 kV/cm. The entire crystal becomes T001 monodomain near 33 kV/cm. After the E field is removed, the crystal does not reestablish the T100 and T010 macrodomains, but instead breaks up into tetragonal microdomains.