In this work we study the influence of manganese doping on the electromechanical properties of PZN-12%PT single crystal. The full electromechanical tensor of doped PZN-12%PT in the tetragonal single domain state is determined by the resonance-antiresonance method. Doping leads to a decrease in the dielectric transverse permittivity epsilon(T)(11) and of the shear piezoelectric coefficient d(15). We show by dielectric constant epsilon(T)(33) measurements that the single domain state in doped crystal is stable in plates as thin as 90 mu m, whereas it was unstable in plates thinner than 300 mu m for the undoped crystals. This intrinsic effect is discussed by using a volume effect model based on the symmetry conforming principle of point defects. [Ren, Nature Mater. 3, 91 (2004)]. Mn doping forces the stability of PZN-12%PT single domain state, which makes the doped crystal a most suitable candidate than the pure crystal for high frequency ultrasonic medical imaging probe.