This paper reports the preparation of manganese monoboride (Mn50B50) nanocrystals by high-energy dry milling of as-cast bulk ingots. The ingot was fabricated by using an arc-melting technique. The as-made sample exhibits an orthorhombic structure with a saturation magnetization of 130 emu/g and a magnetic transition at 566 K. The effects of milling conditions on the structural and magnetic properties of Mn50B50 were investigated in detail. Fully stoichiometric Mn50B50 nanocrystals were produced with an average crystal size ranging from 23 to 11 nm after 2-7 h of dry milling. The as-milled MnB nanocrystals were found to exhibit soft magnetic behavior at room temperature with near-zero coercivity; saturation magnetization decreased to 85 emu/g with increased milling time because of a disordered spin configuration on the surface. High-saturation magnetization and chemical stability, and a low-cost manufacturing process make nanocrystal Mn50B50 a promising candidate for technological applications such as magnetic hyperthermia, magnetic data storage, and magnetic imaging.