Mg/B4C multilayer provides very high theoretical reflectivity in extreme ultraviolet range near 30.4nm wavelength, while the interface between Mg and B4C layer is very poor. In this paper, Co was introduced into the interface between Mg and B4C as a barrier layer. Mg/B4C and Co/Mg/Co/B4C multilayers were designed, fabricated, and measured for the wavelength of 30.4nm. The thickness of Co barrier layer was optimized. Grazing incidence x-ray reflectance measurements show that the structural quality of Co/Mg/Co/B4C multilayer is improved significantly after Co barrier layer inserting, and the optimum thickness of the barrier layer is 1.5nm.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.