Pb[(Fe2/3W1/3)(0.8)-x(Mg1/3Nb2/3)(0.2)(Fe1/2Nb1/2)(x)]O-3 system perovskite powders (phase purity > 99%) were prepared via B-site precursor routes. Lattice parameter changes were analyzed in terms of B-site cation stoichiometries and sizes. Weak-field radio-frequency dielectric characteristics were investigated to verify the effect of composition modification on Curie temperatures and maximum dielectric constants. Unusual relaxation behaviors of frequency dispersion in the dielectric constant spectra at paraelectric region were observed.