Abstract This paper presents the age hardening behavior of metastable beta titanium alloy Ti–15V–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 150h (h). For duplex aging first step was performed in two different ways: (i) 250°C for 24h (ii) 300°C for 10h; second step was carried in the range 350–500°C, for times up to 150h. The decomposition of alpha 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/10h+500°C/10h and 250°C/24h+500°C/8h. 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/24h+500°C/8h led to a four to five fold improvement in high cycle fatigue (HCF) life compared to single aging at 500°C/10h. 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 10h at 300°C accelerated the α-phase precipitation reaction during aging.