The piezoelectric properties of a Pb[(Mg(1/3)Nb(2/3))(0.68)]Ti(0.32)O(3) binary system single crystal poled along the [001] direction in the rhombohedral phase were investigated under pressures up to 400 MPa at 25 °C. For the transverse electromechanical property, the difference Δf between the resonance f(r) and antiresonance frequencies f(a), the Δf/f(r) and the electromechanical coupling coefficient k(31) value in the k(31) mode with hydrostatic pressure (p) became smaller because of the increase in f(r) and the almost constant f(a) with p. The k(31) value decreased by 16% at 400 MPa. On the other hand, for the longitudinal electromechanical property, the Δf, the Δf/f(r) and the k(33) value in the k(33) mode with p remained almost constant because of the almost constant f(r) and f(a) with p. The changes in the values of the elastic compliances s(11)(E) and s(33)(E) with p were found to be large from the changes in f(r) and f(a) with p. s(11)(E) and s(33)(E) at 400 MPa were estimated to be 35.4 and 75.1 × 10(-12) m(2) N(-1), respectively. A mechanical quality factor Q almost constant with p in the k(33) mode in contrast to the large decrease in Q in the k(31) mode with p in the pressure range up to 200 MPa was observed. A k(33) value almost constant with p is considered, on the basis of the engineered domain concept, to be due to the stable domain configuration responsible for the longitudinal k(33) mode. Furthermore, the superior piezoelectric properties of the rhombohedral [001] single crystal in the vicinity of the morphotropic phase boundary composition were recently pointed out to come from the large shear piezoelectric constant d(15) of their single domain property. The hydrostatic pressure cannot influence the piezoelectric properties from the viewpoint of the contribution of the large shear mode d(15), since the uniform pressure introduces no shearing stresses. Consequently, the k(33) value measured for the k(33) mode remained almost constant with p in the measured pressure range.