Lead nickel niobate (Pb(Ni1/3Nb2/3)O3) has been prepared by a newly developed hydrothermal process. During the hydrothermal reaction at 250°C, a pyrochlore phase is formed. After calcining the 250°C-hydrothermally derived precursors at 750°C, a monophasic Pb(Ni1/3Nb2/3)O3compound is successfully produced. Increasing the hydrothermal temperature significantly facilitates the formation of the perovskite phase. In comparison with the solid-state reaction, the hydrothermal process not only reduces the temperature for synthesizing Pb(Ni1/3Nb2/3)O3, but also decreases its particle size to the submicron range. The dielectric properties of Pb(Ni1/3Nb2/3)O3strongly depend on the electric field frequency. Increasing the field frequency results in an increase in the apparent Curie temperature, which is associated with a decrease in the maximum dielectric permittivity. The critical exponent and diffuseness calculated by a modified permittivity-temperature equation verify the relaxor characteristics of Pb(Ni1/3Nb2/3)O3.