Results of two test programs (TP-1 and TP-2) are presented here to investigate: (1) the transition from Regime I to Regime II fatigue-crack growth behavior in a room-temperature high-humidity environment (TP-1); and (2) elevated-temperature fatigue-crack growth behavior of Ti–6Al–4V alloy (TP-2). Constant-amplitude tests were conducted for both at several stress ratios (R=0.05, 0.4 and 0.7) to study the transition of structure-sensitive fatigue crack growth in TP-1, and several temperatures were studied at R=0.1 in TP-2. Middle-tension (MT) specimens were used for the former case, whereas compact-tension specimens were used for the latter. Conventionally forged Ti–6Al–4V alloy processed to a solution-treated and over-aged condition was used for both programs. Temperatures for the TP-2 were: room temperature, 175, 230, 290 and 345 °C. An increase in stress ratio and temperature lowered the transitional stress intensity factor range where Regime I shifts to Regime II (steady-state region). A higher stress ratio accelerated the fatigue crack growth rates. However, higher temperature up to 345 °C influenced the crack growth rates only marginally. An empirical correlation equation was developed and validated to predict the transitional stress intensity factor for a number of materials. Distinct transition and crack growth behaviors were recorded for each stress ratio and temperature for TP-1 and TP-2, respectively.