Titanium alloys are the one of promising candidate materials for medium high temperature parts in the aircraft, automobile, petrochemistry and electrochemistry because of their high strength with low density in medium high temperature. In this study, the effects of aging and heat treatments on the mechanical properties of Ti-15-3 alloy in medium high temperature, which was $400^{circ}C$, were studied. Solid solution treatment was performed at $8000^{circ}C$ of $eta$ phase region for 1 h and the alloy was quenched in water. The alloy was aged at $5000^{circ}C$ of $alpha$ and $eta$ two-phase region for 1, 2, 4, 8, ... and 100 h to increase the mechanical property. The $eta$ single phase was observed at all parts of specimens in Ti-15-3 alloy after ST. As the aging at $500^{circ}C$, fine precipitates of a phase was generated from matrix of $eta$ phase and the microstructure was consisted of weaving structure such as Widmanstiitten a phase. The most suitable aging time is 24h in$ 400^{circ}C$. At this time, strength is 1164 MPa and elongation is about 12%. In room temperature, elongation of Ti-15-3 alloy aged at $500^{circ}C$ for 16 h is poor (=3%) in spite of high tensile strength (1458 MPa).