ReB2-type hexagonal Osmium diboride (OsB2) has been predicted to exhibit higher hardness than its orthorhombic phase, but hexagonal-orthorhombic phase transformation occurs at temperature higher than 600 degrees C, resulting in the decrease in its hardness. Therefore, ReB2-type hexagonal OsB2 samples with Re addition were produced by a combination of mechanochemical method and pressureless sintering technique, and the effects of Rhenium (Re) addition on phase composition, thermal stability and mechanical properties of OsB2 were investigated in this study. X-ray diffraction (XRD) analysis of the as-synthesized powders by high-energy ball milling indicates the formation of hexagonal Os1-xRexB2 solid solution with Re concentration of 5 and 10 at.% without forming a second phase. After being sintered at 1700 degrees C, part of the hexagonal phase in OsB2 transformed to orthorhombic structure, while Os0.95Re0.05B2 and Os0.9Re0.1B2 maintained their hexagonal structure. This suggests that the thermal stability of the hexagonal OsB2 was significantly improved with the addition of Re. Scanning electron microscopy (SEM) photographs show that all of the as-sintered samples exhibit a homogeneous microstructure with some pores and cracks formed throughout the samples with the relative density >90%. The measurements of micro-hardness, nano-hardness, and Young's modulus of the OsB2 increased with Re addition, and these properties of the sample with 5 at.% addition of Re is higher than that with 10 at.% Re.