The microwave propagation properties of a high Tc superconducting transmission line in the mixed state are theoretically investigated by using the electrodynamic field equations for a type-II superconductor developed by Coffey [J. Low Temp. Phys. 108, 331 (1997)]. We have numerically examined the attenuation constant and phase velocity as a function of temperature, frequency, and static magnetic field. The role played by the vortices in the superconducting transmission line is illustrated and discussed.