Abstract: Aimed at solving the problems in the determination of fatigue crack growth threshold of metal materials, such as longer testing period, larger specimen dimension and lower measurement accuracy in crack propagation length, an in-situ observation method was used to determine the fatigue crack growth threshold of TC4 titanium alloy. By this scanning electron microscope in-situ observation technology, the measurement accuracy of crack growth length could be improved and the testing period could be shortened. The results show that the fatigue crack growth threshold of TC4 titanium alloy tested by this method was consistent with the data that had been reported, and the measuring time was much less compared with the traditional method. This implied that this method was an effective method in determination of fatigue crack growth threshold and can be used as an ideal method to determine the fatigue crack growth threshold of metal materials.