Mo/Al multilayers have the potential to be used as reflective mirrors in telescopes for cosmic and solar observation in a large spectral range of extreme ultraviolet radiation. Two types of Mo/Al multilayers were deposited by a magnetron sputtering system for comparison: one used pure Al and one used Al doped with 1.5wt. % Si. The structural performance of these two multilayers were characterized by a X-ray diffractometer, atomic force microscope, and optical profilometer. The angular dependence of the reflectivity was measured using a laboratory-based reflectometer at 58.4 nm. Grazing incidence X-ray reflectivity analysis revealed that the Mo/Al (1.5wt. % Si) multilayers possess a significantly improved interfacial structure compared with Mo/Al (pure). Surface morphology observations of the samples indicated that the incorporation of Si smoothes the surface and reduces the surface roughness. X-ray diffraction results suggest that the Si slightly reduces the average grain size of Al and increases the average grain size of Mo, but their crystallinity is improved. Based on the improvement of structural performance, a peak reflectivity of 29.4% at 58.4 nm is achieved for Mo/Al (1.5%Si) multilayers, whereas it is only 17.4% for Mo/Al (pure).