To crarify the effect of hydrogen on the fatigue sterngth of low-alloy high strength steel SCM440H, fatigue tests were done in hydrogen chrage environment. Vaccuum quenching was appllied to the material so that the effect of hydrogen is appeared clearly. Three kinds of fatigue tests, which are unchraged, hydrogen charged before fatigue test and charged continuously during fatigue test, were done. Hydrogen chrage was done using cathodic chrage method. Internal fatigue fracture was caused in all specimens regardless of chraged or uncraged. Fatigue life of hydrogen charged material decreased comapred with that of uncharged one. in case drastically decrease of fatigue life was whosn, relatively large inclusion was observed at the crack origin. In case of small decrease of fatigue life was shown, no inclusion was observed at the crack origin.
