Polarized Raman spectroscopy has been employed to investigate the evolution of the microstructure of 0.67PbMg1/3Nb2/3O3-0.33PbTiO3 (PMN-33%PT) single crystal in the temperature range from −195 to 300 °C. The MA-MC-cubic transition sequence was observed in the microareas with MA-type (space group Cm) and MC-type (space group Pm) monoclinic structures. Interestingly, the MA-MC phase transition temperature exhibited remarkable microareal dependence due to the spatial inhomogeneity of polar nanoregions (PNRs). The MC-cubic phase transition took place at 155 °C in both microareas, which consisted well with previous reports. These results reveal that the phase transition in PMN-33%PT single crystal is closely related with the thermal dynamics of PNRs, which will be useful for understanding the microheterogeneity in this compound.