Increasing frequency is always an important development direction of high power microwave (HPM). However, as the frequency increases, the volume of an HPM device decreases rapidly and so does the power handling capacity (PHC). For obtaining reasonable PHC, the design of a V-band coaxial transit time oscillator (TTO) based on TM₀₃ mode is carried out to achieve low surface field in this paper. The slow wave structure (SWS) having a large radial width with TM₀₃ mode in such a device designed will lead to low risk of both electric field breakdown and pulse shortening. Firstly, the synchronous effect of velocity between microwave and electron beam is achieved by calculation of dispersion curve and coupling impedance. Then, small group velocity and high coupling impedance are designed to make TM₀₃ a dominant mode in the coaxial V-band TTO. Finally, under the condition of 440 kV and 5 kA, an HPM is generated to reach an output power of 440 MW with a low surface electric field of 1.6 MV/cm, a microwave frequency of 62.25 GHz, and a beam-to-wave efficiency of 22% from numerical simulation.