Effects of calcining and sintering temperatures on the microstructure and the microwave dielectric properties of Ba6-3x(Sm0.2Nd0.8)8+2xTi18O54(x=2/3) ceramics were studied. When the calcining temperature was lower (1373K), the dielectric constant ε and the Qf factor initially increased with increasing sintering temperature, but decreased when sintering temperature was higher than 1603K. As a whole, the temperature coefficients of the resonant frequency τf tended to increase with increasing sintering temperature. The microstructure of the sintered ceramic consisted mainly of columnar and block-shape grains. The grain size increased with increasing sintering temperature. When the calcining temperature was higher (1473K), εr and τf were nearly independent of sintering temperature. The excellent microwave dielectric properties were achieved in the Ba6-3x(Sm0.2Nd0.8)8+2xTi18O54 (x=2/3) ceramic calcined at 1373K and sintered at 1603K for 3 h: ε = 80.8, Qf = 8114 (GHz) and τf = 5.6.