Abstract: The telephoto microscope observation method,silver coating method,acoustic emission technique and strain gauge test method have been used for on-line NDT in the low cycle fatigue test of TC18 titanium alloy specimens.Comparing the detection results of the initial crack and the crack length,it shows that the strain gauge test method should be used to detect the initial crack and the telephoto microscope observation method can be used to observe the crack length in low accuracy;otherwise,the silver coating method is the best choice.