In this paper, we report the design methods and numerical results for a kind of high-power microwave (HPM) circular-waveguide transmission line, which has a 90deg bend to change the wave propagating direction for certain purposes and, meanwhile, also acts as a TM01-TE01 converter, transforming the HPM sources generating TM01 mode into the TE01 mode. Two types of mode-conversion sequences, i.e., TM01-TM11-TE01 and TM01-TE11-TM11-TE01 , are proposed. In both sequences, a 90deg-bend TM11-TE01 mode converter with sinusoidal curvature is adopted. Besides, in the first sequence, a new configuration of serpentine TM01-TM11 converter is proposed, which provides high conversion efficiency with short length; in the second one, compact linear TM01-TE11 and TE11-TM11 converters are designed. These two types of transmission lines have similar transmission efficiencies of 98.2% and 98.6% (ohmic attenuation in Cu waveguides is included), respectively, at 3.5 GHz, with bandwidths (for mode-conversion efficiency >90%) of 6.0% and 5.7%, respectively.