Internal friction in two kinds of iron-manganese alloy which contain Mn 17.5% and Mn 12.8% respectively and one kind of copper-aluminum alloy containing 13% Al were measured with a torsion pendulum, and internal friction peaks were observed in the temperature range in which the martensite or reverse martensite transformation is taken place. The condition for the appearance of this internal friction peak is that it occurs only when accompanied by the process of martensite-type transformation.Systematic observations were made with iron-manganese alloy containing 17.5% Mn, and it was found that the height of internal friction peak increases with the increase of the rate of heating (or cooling) or the stress, decreases with the increase of the frequency of vibration or the carbon content in the specimen. All these may be explained by the reason that the more the amount of new phase has transformed in one period of vibration, the larger is the internal friction.The mechanism of the internal friction peak was discussed. The conclusion was reached that because of the suddeness of the formation of martensite, the applied external forces due to torsion pendulum may be considered as constant during the time of formation for martensite plates, and since half of the work done by the external forces during the alteration must be lost as confirmed by the calculation of interaction energies between stresses and iuhomogenities, it consequently gives rise to the substantial portion of internal friction. On the other hand, the stress-induced phase transformation also gives rise to a minor portion of internal friction.