A special-velocity-matched electro-optic (EO) phase modulator employing a microwave resonant design in lithium niobate is presented. Both the microwave property and phase-modulation performances of the 3.25-GHz modulator agree well with the numerical simulations. Using this modulator in a single-pass configuration with 1-kW microwave drive power, the spectral bandwidth of a 1 053-nm, 3-ns pulse-length laser is broadened to 0.13 nm. With a clear aperture of 5×5 (mm), the modulator is suited for two-dimensional smoothing by spectral dispersion in high power laser systems.