Internal friction in hardened carbon steels was measured with a torsion pendulum and an internal friction peak was observed around 130℃ when measurements were taken from room temperature upwards. This peak disappeared completely after the temperature of the specimen reached 170℃. This phenomenon was observed in carbon steels containing carbon ranging from 0.29% to 1.4%, and also in an alloy steel. The appearance of this internal friction peak seems to indicate that the transformation product (ε-carbide) formed in the first-stage tempering of martensite is in coherence with its parent phase, and the origin of internal friction is the stress-induced movement of the plane of coherence.The above-mentioned internal friction peak was not observed in 0.25% carbon steel specimens having a martensite structure. However, after such a specimen has been tempered at a temperature around 300℃, an internal friction peak was observed around 150℃. This indicates that the transformation product formed in martensite containing 0.25% carbon in the third-stage tempering is in coherence with its parent phase. Since the internal friction peak associated with this transformation product behaves differently from that associated with this ε-carbide, so it may be concluded that this transformation product is not ε-carbide.