Abstract: The opinion has been existing for a long time that the bandwidth of mode converters will increase as the converter length is shortened when the conversion efficiency remains the same at the central frequency. However this paper indicates that there is an optimum length of TE01-TE11 mode converter. When the length of the mode converter is longer than the optimum length, the bandwidth decreases as the length increases; when the length of the mode converter is shorter than the optimum length, the bandwidth increases as the length increases. It is verified by the design of the TE01-TE11 mode converter whose central frequency and radius are 35 GHz, 13.6 mm respectively. The optimum length is 550 mm, and a bandwidth reaches a maximum of 6.28%.