Abstract: Hard α defect is an inevitable defect introduced in TC4 titanium alloy metallurgical process.Its properties are quite different from those of titanium alloy matrix, so it has become an important factor affecting the reliable use of titanium alloy components.In order to investigate the influence of hard α defect on the fatigue life of titanium alloy specimens and overcome the difficulty that the size of hard α defect can not be detected in metallurgical process, TC4 titanium alloy specimens with built-in hard α defect were prepared and low cycle fatigue tests were carried out.The inclusion size is equivalent to a crack by area equivalence method, and Franc3D is used to calculate the crack growth life and subtract from the total life of the test piece.Thus, the crack initiation life of the hard α defect test piece with different nitrogen content is obtained.From the analysis of the test results, it can be concluded that the fatigue life of the crack significantly reduces by the hard α defect, but there is a threshold of nitrogen content, which can make the crack initiation life increase when the nitrogen content is higher.