A novel mode converter that transforms coaxial transverse electromagnetic (TEM) mode to TE11 circular waveguide mode is investigated. It is shown that with four metal plates inserted in the coaxial waveguide, the TEM mode will be converted to coaxial TE11 mode, and finally, it forms a TE11 conventional circular waveguide mode. The experimental test results are in good agreement with the computer simulations, which demonstrates the feasibility of the mode converter.