The first-order electric field-induced phase transition in Pb(Mg_1/3Nb_2/3)O3, [PMN] single crystals, from the average cubic phase to a macroscopically polar 3m phase, was studied more specifically by the optical domain examinations with polarized light microscopy. Different aspects of induced macrodomain structure and phase boundary related to the establishment, decay and reversal of the induced polarization were presented. The mechanism of the induced phase transition was described, from the domain point of view, as a transformation from polar nanodomains to polar macrodomain states resulting in a trigonal distortion with symmetry breaking.