Ductile Ni3Al-B type intermetallic alloys show a unique fatigue crack growth behavior at elevated temperatures. A crack propagation mechanism has been investigated in an experimental P/M alloy by testing the alloys with different fatigue frequencies at 400°C. The Ni3Al-B intermetallic alloy shows a substantial time-dependence of fatigue cracl growth rate when tested in air. Under a given cyclic stress intensity, an order of magnitude difference of crack growth rate was observed by decreasing the fatigue frequency. However, such a time-dependence did not occur when the alloy was tested in vacuum. It is concluded that “dynamic” embrittlement in an oxidation environment is the major factor controlling the fatigue crack growth in ordered Ni3Al-B alloys at elevated temperatures.