Internal friction in an iron-manganese alloy containing 18.5% Mn was measured by a torsion pendulum; a stable internal friction peak was observed within the temperature range for the direct as well as for the inverse martensitic transformation of this alloy. When measurement was taken in descending temperatures, the internal friction peak associated with the direct martensitic transformation appeared round 100℃; and that associated with the inverse martensitic transformation appeared round 200℃ when measurement was taken in ascending temperatures.Systematic studies were made on these two internal friction peaks after they became stable and they were found to have the following characteristic features:1. The position of the peak is independent of the vibration frequency used;2. The height of the peak increases appreciably with an increase of the strain amplitude used in the measurement of internal friction;3. With an increase of the number of cycles of heating and cooling given to the specimen, the position of the peak shifts to a temperature further apart from the initial transformation temperature, and the height of the peak decreases consecutively.The independence of the position of the peak on frequency indicates that the basic process giving rise to internal friction is not controlled by heat activation. The dependence of the height of the peak on strain amplitude indicates further that the mechanism of internal friction is not concerned with anelasticity. Observations by transmission electron microscopy have revealed that the stacking fault associated with an extended dislocation may serve as nuclei for martensitic transformation of the Fe-Mn alloy studied, so that the stable internal friction peaks observed may be associated with the stress-induced expansion and contraction of the extended dislocations in the specimen. Such a model can explain the internal friction phenomena observed in the Fe-Mn alloy in a unified manner.