Abstract: Titanium alloys are widely used in the military aerospace region.However,hydrogen induced delayed cracks could occur in the weld of some titanium alloys in certain welding condition.And the mechanism concerning hydrogen induced delayed crack is not clear.In this paper,the pre cracked CT specimens of Ti55 charged with hydrogen were strained in tension and the effect of hydrogen content was studied on the threshold stress intensity factor K_th,and the crack growth rate da/dt;at a crack tip in EBW of Ti55 alloy.Meanwhile,the mechanism of hydrogen induced delayed crack was analyzed.The results indicate that the solid solubility of hydrogen in Ti55 is about 79&×10^-4%.At hydrogen contents below 79&×10^-4% K_th decreases with increasing hydrogen content,whereas da/dt increasess as the hydrogen content increases.When hydrogen content gets to about 79&×10^-4%, K_th reaches the minimum and keep an invariable value.The mechanism of hydrogen induced delayed crack in EBW of Ti55 alloy is the diffusion of hydrogen up a stress gradinent to form hydride TiH₂ which precipitates at the crack tip.