In this paper, effect of lanthanum doping on the dielectric and piezoelectric properties of the morphotropic phase boundary (MPB) composition in the Pb(Zn1/3Nb2/3)O-3-BaTiO3-PbTiO3 system is presented. Samples were prepared according to the formula Pb-0.85-xBa(0.15)La(x)[(Zn1/3Nb2/3)(0).7Ti(0.3)](1-x/4)O-3 (0 < x < 0.3), where the compensation for La ions was achieved via the appearance of B-site vacancies. All the compositions were synthesized using columbite precursor method and sintered using inverted crucible approach. Results of x-ray diffraction demonstrate that the solid solubility limit of La2O3 in the PZN-based MPB composition is more than 30 mole%. Incorporation of La2O3 into A-site sublattice of perovskite structure stabilizes rhombohedral phase against tetragonal phase, as a consequence, the location of the MPB composition range is displaced toward PT-rich end. In addition, the lattice distortion degree, represented by either tetragonality for tetragonal phase or rhombohedrality for rhombohedral phase, is reduced. Increasing La2O3 content remarkably decreases both the dielectric permittivity maximum and the temperature of the dielectric permittivity maximum. However, addition of La2O3 obviously strengthens the degree of the frequency-dispersion, which is correlated with the weakened degree of ferroelectric couplings among oxygen octahedra. The degree of diffuse phase transition (DPT) is prominently enhanced with increasing La2O3, for which the underlying mechanism is explored. Moreover, the ever-increasing extent of departure from Curie-Weiss behaviour induced by the lanthanum doping is indicative of some changes with the respect to the scale of polar microregions inherent to relaxors. The longitudinal piezoelectric coefficient (d(3)3) is maximized at 1 mole% La2O3, which is attributed to the combined effect of phase coexistence and enhanced contribution from non-180 degrees domain wall process associated with rhombohedral phase.