This paper presents the age hardening behavior of metastable beta titanium alloy Ti-15 V-3Cr-3Al-3Sn subjected to various single and duplex aging treatments. Single aging was carried out in the range 200-550 °C for times up to 150 h (h). For duplex aging first step was performed in two different ways: (i) 250 °C for 24 h (ii) 300 °C for 10 h; second step was carried in the range 350-500 °C, for times up to 150 h. The decomposition of alpha (α) phase during aging was monitored by hardness measurements, tensile testing, different microscopic techniques and X-ray diffraction. Duplex aging treatments resulted in higher hardness and strength compared to single aging. There was a good balance of tensile strength and ductility after two duplex aging treatments 300 °C/10 h + 500 °C/10 h and 250 °C/24 h + 500 °C/8 h. Precipitate free zones (PFZs) were observed when aged at ≤450 °C in the microstructure; aging at ≥500 °C gave freedom from such zones. Duplex aging resulted in a finer size and higher density of α precipitates compared to single aging. Duplex aging 250 °C/24 h + 500 °C/8 h led to a four to five fold improvement in high cycle fatigue (HCF) life compared to single aging at 500 °C/10 h. The observed improvement in HCF life could thus be related to smaller size and larger density of α precipitates resulting from duplex aging. Optical microscopy and X-ray diffraction studies clearly revealed that preaging for 10 h at 300 °C accelerated the α-phase precipitation reaction during aging. © 2014 Elsevier B.V. All rights reserved.