In this paper, we propose a new carrier depletion-type hybrid III-V/Si optical phase shifter using a n-III-V/p-Si hetero-junction, which can be fabricated with direct epitaxial growth of III-V semiconductors on Si. We numerically analyzed the performance of the III-V/Si hybrid optical phase shifter by comparing the performance in reverse bias operation with that of pure Si or III-V p-n optical phase shifters. The hybrid III-V/Si optical phase shifter showed improved modulation efficiency and lower optical loss compared to pure Si and III-V p-n optical phase shifters, owing to the large electron-induced refractive index change of III-V compound semiconductors, while avoiding the large hole-induced optical loss of III-V compound semiconductor. The simulation study suggests the feasibility of a very low voltage-length product (V-pi L) of 0.07 V . cm, a low insertion loss (alpha) of 16 dB/cm, and a very low alpha V-pi L product close to 1 V . dB at 1.31 mu m, which is 10x lower than for Si p-n optical phase shifters.