Abstract: A aperture-coupling directional coupler between rectangular waveguide and circular waveguide through small aperture was designed to measure the output power of high power microwave devices. The formulas of single-hole coupling degree were made out with loose-coupling theory. Numerical computation of aperture was made by MATLAB. The compared results show that the theory was reasonable. Based on the results of single hole coupling, some useful conclusions were derived about multi-hole coupling. Single hole coupler can not realize directional coupling, but even numbers of holes can.The results are ualuable for theoretical research and application as well．