Transition metal osmium borides were synthesized by mechanochemical method using high-energy ball-milling with Os (Osmium) and B (Boron) powders as raw materials. The formation process, reaction mechanism, and thermal stability of the mechochemically synthesized osmium borides were studied. Almost pure Os2B3 phase was obtained when the Os-to-B molar ratio was 1:2; while ReB2-type hexagonal OsB2 with a small amount of RuB2-type orthorhombic OsB2 was obtained when the Os-to-B molar ratio was 1:3. Stoichiometry OsB2 was obtained from boron rich starting mixture powders due to the B loss during the high-energy ball-milling process. It was also found that WC and osmium oxide were present as contaminants after ball milling for 40 hours. Heat treatment results revealed that the as-synthesized Os2B3 powders are thermally stable in flowing Ar up to 800 degrees C, but a transformation from hexagonal to orthorhombic structure partially occurred for the OsB2 powders as low as 600 degrees C.