Abstract: The process of initiation,propagation and rupture of hydrogen induced delayed cracks was investigated by means of acoustic emission(A.E.) technique and implant testing method.By taking the first A.E.signal,or several successive signals,position of initiation of cracking can be sought out with certainty.Experimental results show that in testing implants with circular notch the position of initiation located normally at a point near the intersection of the fusion boundary and the notch.According to the waveform of A.E.,the process can be divided into four stages,i.e.,initiation, delayed propagation,rapid propagation and instantaneous rupture.The microfractography of the different regions shown on the fracture surface,relevantly standing for the different stages,was studied with a scanning electron microscope.As the crack propagates from the delayed-region into the rapid-region and then into the final rupture region,the stress intensity factor K on the tip of the crack gradually increases,while H_D decreases, and the micro-appearance of fracture surface changes from IG+QC_HE to QC+DR to DR. The authors maintain that the delayed propagation stage is the essence of hydrogen induced delayed cracking,therefore the delayed-region of the fracture surface is the most significant one.The path of crack propagation in an implant was investigated by the multiple sample method.In this paper the influence of hydrogen on the micro-appearance of the delayed region of fracture surface was also discussed.The act that implant samples after treatment for hydrogen removal have no or extremely few IG modes on fracture surface,whereas those with low hydrogen content have quite a number of (IG)_DR modes and thase with high hydrogen content suffer from having an increased number of crystal sugar like IG modes obviously indicates the harmful effect of hydrogen on the grain boundary of primary austenite.