Core-shell structures have been found by both scanning and transmission electron microscopy in undoped Pb(Fe 1 / 3 W 2 / 3 )O 3 ceramics when a solid-state-reacted powder of stoichiometric composition was sintered at 800 o C. The presence of superlattice reflections of the {1/2 1/2 1/2}-type, often termed F-spots, indicates that the existence of nanoscale ordered domains of 2-5 nm size in both core and shell is due to chemical ordering of the B-site cations with alternating Fe 3 + and W 6 + along the <111> direction. The ceramic is densified by the assistance of a low-temperature liquid eutectic at ~690 o C. A formation mechanism of core-shell structures in Pb(Fe 1 / 3 W 2 / 3 )O 3 ceramic is proposed. It can be described by solution-reprecipitation and phase transformation subsequently to perovskite Pb(Fe 1 / 3 W 2 / 3 )O 3 , involving at least two intermediate phases which are formed at temperatures below and above the low-temperature liquid eutectic point and related to pyrochlore (Pb 2 FeWO 6 . 5 ). The cores are characterised by ''black dots'' and contain relatively Fe-excess. The shells exhibiting featureless contrast are free of ''black dots'' but contain W-excess relative to the cores. Long term sintering at 800 o C has facilitated the inward diffusion of W 6 + to the grain interior when the distribution of the B-site cations is improved. As a result, the shell increases its size at the expense of the core.