La-doped lead magnesium niobate (Pb1-xLax[Mg(1+x)/3Nb(2-x)/3]O-3, PLMN, x= 0.1, 0.2) was prepared by a polyethylene glycol-based process (PEG method) as follows: PEG-cation (La3+, Mg2+, Nb5+) complex was oxidized and the obtained oxide was mixed with PbO. This oxide mixture was fired to obtain PLMN. This PEG method enabled to prepare the single phase of perovskite structure of PLMN at 1023 K, This temperature is lower than that used in the solid state reaction method. The PLMN prepared by the PEG method had quite little compositional fluctuation and structural strain. PLMN has an ordering domain with the super lattice structure of B site cations in the perovskite structure, The crystallite size of the ordering domain in the PLMN obtained by the PEG method increased when the molecular weight of the initial PEG increased (from 150 to 7500). The formation process of the ordering domain was under the influence of the cation distribution in the PEG-cation complex.