Hair-line cracks or flakes in steel are known to be associated with the presence of hydrogen. A prevailing theory states that the crack formation is due to disruptive hydrogen pressure being built up in defects in steel. However, little was known regarding to the nature of these defects and the detailed mechanism of the hair-line crack formation. An attempt was made in the present research to investigate this problem by means of internal friction measurements. The preliminary experiments show: (1) The presence of hydrogen in certain steels may give rise to the appearance of an internal friction peak when internal friction is plotted against the temperature of measurement; (2) a slight amount of cold-work or soaking at a given temperature has a considerable effect on the optimum temperature of the observed internal friction peak, which indicates that the internal friction peak may be connected with the defects or stress condition in steel; (3) the internal friction peak shows an anomalous amplitude effect, i.e., at a given temperature of measurement an internal friction peak is obtained when the internal friction is plotted against the stress amplitude. Such an anomalous behavior is similar to that previously observed by. one of the authors (Ke) in the case of aluminimum containing 0.5% Cu. It is therefore considered that the internal friction peaks observed may be related to the interaction of hydrogen with the dislocations in steel.