Barium strontium titanate/magnesia (BSTO/MgO) ferroelectric materials for phase shift were prepared by traditional ceramic process-solid phase synthesis. The effects of various rare earth oxides of 0.5% on dielectric behaviors of BSTO/MgO composites were studied in terms of permittivity, loss tangent and tunability both at low and high frequencies. The dielectric constant of Y2O3 and Er2O3 doped samples decreases from 160 to 120, and the microwave loss of La2O3 and Er2O3 doped samples decreases from 8.2×10-3 to 6.8×10-3. Only La2O3 increases the tunability of BSTO/MgO system, from 13.6% to 14.8%. For the La2O3 doped sample, the value of tunability is more than 14% with the external DC field 4000 V*mm-1 and the microwave loss at 2.47 GHz is 6.77×10-3 and, hence, it can basically meet the requirements of phase shifters working at microwave frequencies. The influence mechanism was discussed preliminarily.