Abstract: A novel coaxial directional coupler was designed for L-band high power microwave radiation field measurement. It could incredibly decrease the size of the coupler to use two different kinds of transmission lines and coupling aperture with continuous variation width along the propagation direction. The aperture is determined by infinitesimal method, small hole coupling formulas and numerical full wave simulation optimization. Based on the simulation results, the structure of the coupler is optimized and fabricated, and the measurement results indicate that, the size of the directional coupler is 150 mm46 mm55 mm, and during 1-2 GHz, the coupling is 47 dB, the directivity is more than 17 dB.