This letter presents a simulation-based design of an in-line coaxial-to-circular waveguide transition for converting the coaxial transverse electromagnetic mode (TEM) to the circular waveguide TE11¬ mode over an octave bandwidth. The proposed transition consists of a coaxial-to-rectangular waveguide converter and a rectangular-to-circular waveguide transformer. The dimensions of the transition have been optimized using a commercial simulation tool. The proposed transition operates from 10.02 GHz to 21.90 GHz with reflection coefficient of less than −20 dB and higher-order mode level of less than −27.8 dB.