Design and performance optimization of phase modulators is presented by considering both microwave and optical characteristics. Performance modeling is conducted using OptiBPM for beam propagation method of optical waveguide and finite element method using HFSS for driving electrode design. The optical waveguide core realized using electro-optic (EO) polymer PMMI-CPO1 with a refractive index n1=1.63, EO coefficient reo=70 pm/V, RF dielectric constant ϵr1=3.15, and loss tangent tanδ1=10-3 that is embedded in a cladding polymer of NOA65 with n2=1.51, ϵr2=3.2 and tanδ2=2.2×10-2. A phase modulator with an input half-wave voltage of 8 V and a bandwidth of 43 GHz operating at the wavelength of 1.55 μm for RF and optical interaction length of 1.3cm. © 2015 IEEE.