Based on the general theory of mode coupling and principle of the mode converter with cosinusoidal profile, we design a transition section for 94GHz gyrotron oscillator. Numerical calculation and three-dimension electromagnetic simulation for the suggested transition section are made. The results show that the 94GHz-TE01-mode transition section has transmission efficiency 98.8% and low reflection below −40db within a wide bandwidth 4GHz. It is a timesaving method to design a transition section with the profile by our design.