An analytic expression for the logarithmic decrement is obtained on the assumption that the amplitude-dependent internal friction is due to the resistance to dislocation motion by impurity atoms. A good agreement between the calculated and experimental curves is demonstrated. The dependences of the internal friction parameters on the impurity concentration and plastic deformation are investigated. The results obtained show that the effective force of friction acting on dislocations and its rms deviation can be deduced from an analysis of the amplitude dependence of the decrement.