The dielectric and electrical conductivity properties of xPb(Mn1/3Sb2/3)O-3-(1-x)Pb(Zr0.52Ti0.48)O-3 (PMS-PZT) ceramics were studied. X-ray diffraction indicated that all samples exhibit a single-phase perovskite structure. Dielectric study revealed that the dielectric relaxor behavior was induced by co-doping Mn2+ and Sb5+ into Pb(Zr0.52Ti0.48)O-3 ceramics. Electrical conductivity measurements showed that the concentration of carriers are increased with the increase in PMS contents. After annealing in an oxygen atmosphere for 30 h, the direct current conductivity of PMSZ15 was much lower than that of the as-sintered sample. The reason why this phenomenon occurs may be the reduction of oxygen-vacancy concentration.