Hydrogen absorption into high strength steel bolts and its effects on delayed fracture susceptibility have been investigated, by means of thermal hydrogen analysis on exposed bolts and hydrogen permeation tests under atmospheric exposure. Thermal hydrogen analysis on small parts of exposed bolts showed hydrogen localization near the surface of the bolts. Hydrogen permeation tests under atmospheric exposure showed intermittent hydrogen entry, and maximum instantaneous hydrogen content was bigger than the measured value by the thermal hydrogen analysis. Delayed fracture susceptibility of high strength bolts with tensile strength of 1521 N/mm(2) depended upon the safety index defined as the ratio of the threshold hydrogen permeation coefficient (JL(th)) and the maximum hydrogen permeation coefficient under atmospheric exposure (JL(env)). The results suggest that hydrogen at the surface of the bolts plays an important role on delayed fracture.