Nb and Mn were doped, respectively, to 0.8Pb(Sn0.46Ti0.54)O-3-0.2Pb(Mg1/3Nb2/3)O-3 (PST-PMN) to improve electrical properties for meeting the requirement in various fields. The additions of Nb and Mn influence in a pronounced way the structure, and improve the densities of the ceramics. Nb-doped PST-PMN increased the piezoelectric coefficient d(33), planar electromechanical coupling k(p), and relative dielectric constant epsilon, indicating soft piezoelectric behavior. Mn doping played a hard part, which significantly enhanced the mechanical quality factor Q(m) without deteriorating other piezoelectric properties. The most excellent properties of Nb-doped PST-PMN were obtained with doping amount of 0.75mol%, specifically d(33), k(p), being on the order of 455pC/N, 57.5% and 3560, respectively. The addition of 0.75mol% Mn for PST-PMN presented the optimum electrical properties, with Q(m) of 554, d(33) of 430pC/N, k(p) of 57.0%, epsilon of 2770. It was proposed that the addition of Nb, Mn generated different defect dipoles involved in the domain walls motion and intrinsic piezoelectric responses, leading to different effects on electrical properties.