A systematic study of the temperature dependence of superlattice reflections in the reciprocal space of a single crystalPb(Mg1/3Nb2/3)O3(PMN) was performed using anomalous x-ray scattering technique. Our studies confirm the existence of the two types of nanoregions in PMN: a) chemically ordered nanodomains, and b) polar nanodomains formed by short-range correlated ionic displacements. No detailed temperature dependence of these superlattice reflections has been reported in the past. From monitoring the superlattice reflections corresponding to polar nanodomains (i.e., &agr; spots) over the temperature range 15 K–800 K, we found freezing temperature(Tf)of correlated atomic displacements was near 200 K. In contrast, chemically ordered nanodomains, which give rise to F type superlattice peaks, exhibited spherical shape over the entire temperature interval 15 K–800 K. This leads us to suspect that chemically ordered regions might be different and independent of polar nanodomains. In this paper we explain the contributions of different atomic displacements to &agr; and F superlattice reflections in PMN. Correlation and competition between Pb displacements and the tilting of oxygen octahedra network are presented to understand the microstructural origin of the relaxor ferroelectric behavior. ©2000 American Institute of Physics.