Abstract: A serpentine TM01-TE11 mode converter with input and output ports aligned on the same axis was designed by adopting phase rematch technique. The converter is formed with three constant-curvature bent waveguides and two straight waveguides. Numerical investigations on this mode converter were presented with a general optimizing code employing coupled mode theory. A mode converter of this kind centered at 7.0 GHz was designed. The efficiency of the conversion from TM01-TE11 exceeds 99% at 7.0 GHz and 90% over the frequency range of 6.5 to 7.5 GHz. Simulations with FDTD and FEM methods verified the theoretical designing results respectively.