Titanium (Ti) alloys have been used in several industrial applications owing to their unique mechanical properties and superior corrosion resistance. The β type alloys are one of the most functional classes of Ti alloys in relation to the processing, microstructure and mechanical properties. The composition of Ti alloys is influenced by the fabrication method and alloying composition. In this work, the effect of alloying element Sn on the mechanical hardness of as-cast metastable β type ternary alloys Ti-30Nb-xSn was studied. Sn was expected to stabilize the β phase. The Sn concentration in the alloys varied 2, 5, and 8 wt%. The alloys hardness was measured by Vickers Hardness test. The microstructure and chemical composition were investigated by scanning electron microscope (SEM) and x-ray diffractometer (XRD), respectively. The results indicated that the hardness was altered by the Sn content in the alloys. The β phase was stabilized at a concentration of alloying element Sn 5 wt%. Meanwhile, at both low and high Sn concentrations the α-phase coexisted with the β phase. As a consequence, the Ti-30Nb-5Sn exhibited the lowest hardness (455 HV) relative to the Ti-30Nb-2Sn (473 HV) and Ti-30Nb-8Sn (559 HV) alloys.