Sub-terahertz (THz) frequency is attracting extensive interests for a wide range of applications. Electronically tunable phase shifters as one of the key building blocks in sub-THz systems, especially waveguide based phase shifters, encounter difficulties in both design and fabrication. In this paper, an electronically tunable rectangular metallic waveguide with graphene integrated operating in sub-THz frequency is proposed and its phase modulation properties are investigated. It shows that a maximum 34-degree linear phase shift is obtained at 315 GHz with bandwidth more than 50 GHz, and the reflection and maximum insertion loss are lower than -15 dB and 3.5 dB in this frequency range, respectively. The phase modulation feature can be applied to design of graphene based waveguide phase shifters and other graphene based waveguide phase modulators.