Detailed theoretical investigations on the nature of internal friction of linear type have been made by Zener and others. It appears that their results are not systemmatic enough owing to the lack of generality. In particular, very little has been attempted to the illucidation of the nature of mutual interactions due to causes which give rise to internal friction.In this paper a theory is proposed based on the general theory of thermodynamics ol irreversilc processes together with the Boltzmann superposition principle. A comparison is made of the results of both theories. It is found that definite variance exists in the instances in which the mutual interactions become predominant. It would therefore appeal to further experimental demonstrations. In view of the recent rapid progresses in the experimental studies of anomalous and amplitude dependent internal frictions, it would be of interest to carry out a programm of study of the internal friction of non-linear type by the procedures similar to that proposed in the text.