Delayed fracture of medical Ti devices was studied by micro-mechanics. Effects of oxygen and hydrogen on the characterization of titanium were discussed in terms of macro and microscopic aspects. Hydrogen can have a major effect on the microstructure and properties of Ti. When the hydrogen concentration exceeds the terminal hydrogen solubility, a titanium hydride phase (TiH₂) can begin to form. Transformation from α-Ti to the hydride phase results in a volume expansion. Hydrides are formed along the grain boundary deeply in the surface. Micro-cracks are formed on the hydrides when an external force is applied. The cracks are developed and accelerate by the mechanism of stress-corrosion cracking related to hydrogen. Experimentally, we confirmed this by fatigue tests of pure Ti and by delayed fracture tests under sustained tensile loading in fluoride ion solutions. The results indicate that stress-corrosion cracking is an effective factor in delayed fracture of Ti.