Tin (Sn) substitution into the B-site and Nd/Sn cosubstitution into the A- and B-sites were investigated in a Ba6-3xSm8+2xTi18O54 solid solution (x=2⁄3). A small amount of tin substitution for titanium improved the temperature coefficient of resonant frequency (f) but led to a decrease of the relative dielectric constant (з) and the quality factor (Qf). The Ba6-3xSm8+2x(Ti1-zSnz)18O54-based tungsten-bronze phase became unstable for compositions with a tin content of >10mol%, where BaSm2O4 and Sm2(Sn,Ti)2O7 appeared, and finally, these phases became the major phases. On the other hand, Nd/Sn cosubstitution led to a good combination of high ε, high Qf, and near-zero Tf. Excellent microwave dielectric properties were achieved in Ba6-3x(Sm1-yNdy)8+2x(Ti1-zSnz)18O54 ceramics with y=0.8 and z =0.05 sintered at 1360℃ for 3h:=82, Qf=10 000GHz, and calculated Tf=+17 ppm/℃. The tolerance factor and electronegativity difference exhibited important effects on the microwave dielectric properties, especially the Qf value. A large tolerance factor and high electronegativity difference generally led to a higher Qf value.