The mechanical properties of the various heat treatment conditions on Ti-15Mo-3Nb-3Al-0.2Si alloy plates were examined. XRD patterns from the surface of Ti-15Mo-3Nb-3Al-0.2Si were analyzed as a solution-treated Ti alloy has the single-phase β structure whereas the aged Ti alloys have the β matrix embedded with α needles. High strength (~1500 MPa) with decent ductility (7%) was obtained by the Ti alloy double aged at 300℃ and 520℃ for 8 hours each. The double-aged alloy exhibits the finer structure than the single-aged alloy at 300℃ for 8 hours because of the higher nucleation rate of α needles at an initial low aging temperature (320℃). TEM observation revealed that the fine nanostructure with α needles in the β matrix ensured the excellent mechanical properties in the double aged Ti-15Mo-3Nb-3Al-0.2Si alloy. In the solution treated alloy, the yield drop, stress-serrations and the ductility minimum typically associated with dynamic strain aging can be attributed to the dynamic interaction between dislocations and oxygen atoms. The yield drop and the stress serration were not observed in aged samples because the geometrically introduced dislocations due to phase precipitates suppressed the dynamic strain aging.