To investigate the effect of hydrogen on fatigue life characteristics and crack growth behaviors through the entire fatigue life of a carbon steel, tension-compression fatigue tests and elasto-plastic fracture toughness tests were conducted in a hydrogen gas environment under the pressures of 0.7 and 115 MPa. The fatigue tests revealed that the fatigue life and fracture morphology vary drastically with the hydrogen gas pressure. This study demonstrates that such differences can be explained by the combination of fatigue crack growth properties and fracture toughness properties in hydrogen gas at each pressure. (C) 2017 Elsevier Ltd. All rights reserved.