The analyses of the simulation of a single mode buried waveguide optical phase modulator based on SOI material are here reported. The structure has been simulated by Athena from Silvaco simulation package. The buried waveguide is created by doping phosphorus with concentration of 10e15 cm-3 into the substrate. The real refractive index and the absorption coefficient of the waveguide are changed using the free carrier dispersion effect via carrier injection of a pn junction. The efficiency, VπLπ is calculated and the performance is compared with that of the rib waveguide optical phase modulator of the same material and dimensions. Simulation shows that the device can be an efficient device for application in intensity modulation.