The effects of silicon addition on microstructures and tensile properties for Ti-6Al-4V alloy casting were investigated. And the possibility of microstructure control for Ti-6Al-4V-Si alloys by hot-working was discussed. Fine blocky Ti5Si3 intermetallic compounds were precipitated during solidification and cooling at silicon bearing alloys. When silicon contents exceeds 1.96%, plate-like compounds were observed around grain boundaries. Lamella structures were formed at 2.75%Si alloy by eutectic reactions from liquid to beta and Ti5Si3. The solvus temperature of Ti5Si3 was measured by microstructure's observation after solution treatment. Its temperature rose rapidly by silicon addition, and reached to 1400°Cat 2.75% silicon bearing alloy. Lamella structures were solved in beta matrix at equilibrium condition. Silicon addition is effective for beta-grain refinement. Tensile strength was increased rapidly by silicon addition to 0.93%, however, when silicon contents exceeded 1.96%, unstable brittle fracture at a low stress level occurred due to grain boundary precipitates. Fine and homogeneous spheroidized precipitates were obtained by soaking and hot working. The combination of strength and ductility was improved by soaking and hot working. Good combination of strength and ductility was especially obtained in 0.90% silicon bearing alloy which was hot-worked at 950°C. Tensile strength and elongation of that alloy were 1338MPa and 10%. Refinement of micro-structure and improvement of mechanical properties by thermo-mechanical treatment will be possible furthermore.