Abstract: Design and fabrication of the X-ray supermirror used as reflective optical element in Kirkpatrick-Baez (KB) microscope were proposed. W/B4C was selected as coating materials pair. Design method and optimization of X-ray supermirrors were discussed. The designed multilayer structure was deposited on the Si wafer substrate using magnetron-sputtering system. The reflectivities of W/B4C supermirrors were measured by X-ray diffraction instrument (8 keV). The reflectivities of supermirrors are 20% at 1.052° and 1.143° designed grazing incident angles. The bandwidth of reflective plateau reaches 0.3°, which can fulfill the requirements of KB microscope.