The internal friction of Armco-iron in the process of plastic deformation was measured by a middle torsion pendulum on a modified tensile testing machine. The effects of strain rate (in the range of 0.73×10-6/sec to 50×10-6/sec), frequency (0.30/sec to 3.6/sec), annealing and carbon content on the internal friction were studied. The results showed that on the yielding plateau, the internal friction did not change with the strain, but increased linearly with the increase of strain rate and varied inversely with frequency. In comparing the data with the theoretical expression derived from the dislocation dynamic model of internal friction in the process of plastic deformation, it is shown that the dynamic model is in good agreement with the experimental results. The dislocation dynamic exponent m of Armco-Fe derived from this model is 10.8-12.3, which is reasonable.The range within which the dislocation dynamic model holds is also discussed.