To develop high strength steel with high resistance to hydrogen embrittlement, it is necessary to clarify the mechanism of hydrogen entry into steel. Hydrogen entry is caused by corrosion reaction under atmospheric environment. However, the relationship between hydrogen entry and corrosion behavior, and the influence of environmental factors on the hydrogen entry such as temperature, relative humidity（RH） and salt deposition are not fully understood. In this study, simultaneous monitoring of both hydrogen entry and corrosion rate was employed to investigate the influence of environmental factors on hydrogen entry and corrosion behavior. The monitoring was conducted under the atmospheric exposure test.As a result, hydrogen entry and corrosion rate increased with increase of amount of salt deposition. Corrosion rate increased with increase of RH, whereas hydrogen entry had a maximum at medium RH. The mechanism of promotion of the hydrogen entry at medium RH range was assumed to be related to lowering pH due to hydrolysis of Fe3＋ in high ［Cl－］ environment.