The purpose of the work is to assess the possibility of constructing an entropy criterion for the strength of materials by the example of a viscoelastic material under creep and cyclic loads. Methodology and Approach. Study of the entropy criterion for the strength of materials using a mathematical apparatus. The results. The possibility of constructing an entropy criterion for the strength of materials is estimated by the example of a viscoelastic material under creep and cyclic loads. It is shown that the increment of entropy density is determined by the nature of the loading process. For this reason, it turns out to be impossible to propose any value of the limiting value of the increment of entropy density as a criterion for the long-term strength of a viscoelastic material. Theoretical and practical relevance: the obtained results can be applied in the theory of mechanics of composites.