Optimum design of an overmoded TEM coaxial to TE10 rectangular waveguide mode converter at S-band using HFSS software and network theory is presented. The coaxial waveguide has such structure parameters that the TEM,TE11 and TE21 modes can propagate in it . The mode converter can be applied to the power extraction system of HPM sources. The method presented in this paper can be used to optimize the mode converters from coaxial and circular waveguides to rectangular waveguides which are matched with short pistons.