Effects of microstructure, stress ratio, and environment on the fatigue crack growth resistance of Ti-23Al-9Nb-2Mo-1Zr-1.2Si and Ti-23Al-11Nb-0.9Si (at.-%) Ti3Al based alloys have been studied at room and elevated temperatures. Only modest effects of microstructure on fatigue crack growth resistance have been obtained at room temperature, and these tend to reduce further at the elevated temperatures of 600 and 700°C both in air and in vacuum. At room temperature the fatigue crack growth resistance of Ti3Al based alloys is controlled primarily by the thickness of the retained β phase rather than by its volume fraction and the microstructure with a larger average thickness of retained β laths shows improved fatigue crack growth resistance. However, in some microstructures, the spatial distribution of the β phase can also be deduced to be important. A marked difference on crack growth resistance is obtained for stress ratios of 0.1 and 0.5 both at room temperature and at a temperature of 600°C. The mechanisms of fatigue crack growth in air and vacuum are discussed.