Abstract: Calculation expressions of technical parameters are derived for the waveguide-stripling-coaxial line directional coupler. The impact of structure dimension of the coupler and input frequency on the parameters is studied using Matlab. Numerical results indicate that, included angle has the greatest impact on coupling coefficient and directivity; optimal directivity angle are most sensitive to coupling hole depth, and it varies sharply with input frequency in low frequency range (less tham 2.0 GHz) and becomes much smoother when the frequency goes higher. Thus in the design of this coupler, designed coupling coefficient and directivity could be achieved through adjusting stripline size and coupler position besides the conventional ways of changing aperture size and rotation direction.