nalytical formulae for the coupling coefficient β of a waveguide-traveling wave structure coupling system have been established and compared with experimental results. The dimension of the coupling aperture on the coupler cavity of a traveling wave structure can be determined easily. The relation between the coupling coefficient β and the other parameters. such as aperture dimension, wavelength, group velocity, wall thickness and coupler dimension, is explicitly revealed. These formulae include that for the caes of a waveguide-single cavity coupling system which has been established in ref. [1]