In this letter, we present our investigation of elastic, piezoelectric, and dielectric performances in 0.70Pb(Mg 1/3 Nb 2/3 )O 3 –0.30PbTiO 3 (0.70PMN–0.30PT) single crystals using a resonance technique. The complete set of constants was determined for this [001]-poled domain-engineered system by the effective tetragonal 4-mm symmetry. The electromechanical coupling coefficient k 33 , k 31 , k 31<110> , k t and k 15 are 92%, 89.5%, 76%, 62%, and 55%, respectively. The 0.70PMN–0.30PT crystal, which is slightly away from the morphotropic phase boundary (MPB), has remarkable and stable piezoelectric properties and becomes one of the premier considering materials for present applications in making high-sensitivity medical ultrasonic transducers and large displacement actuators.