Integration of an optical waveguide array using four waveguides in a Mach–Zehnder (MZ) structure and a two-layer low-loss substrate integrated waveguide (SIW) structure is presented to propose a new class of electrooptical phase modulators. In the proposed SIW modulator, synthesized rectangular waveguide in planar form is used as “electrodes” to guide millimeter-wave signals instead of conventional coplanar waveguide (CPW) for an electrooptical modulator, thus providing low-loss non-TEM mode propagation. In this study, two layers of a lithium niobate (LiNbO ) substrate for creating a high field interaction between the millimeter-wave and optical signals are considered and studied. Fabrication of the proposed structure by titanium diffusion in an LiNbO substrate as the optical waveguides and laser micromachining of the SIW via-holes has been done. The validation of our simulated results is also performed by the measurement of CPW-to-SIW transitions in the LiNbO substrate. Optical loss of 0.8 dB/cm at the wavelength of 1550 nm is obtained for an MZ structure using an optical waveguide array consisting of four waveguiding channels. In addition, it is shown that 5-GHz bandwidth over a 60-GHz operating frequency range can easily be obtained for the proposed modulator.