Based on the analysis of various existing criteria, this article proposes a new thermodynamic performance criterion, the cogeneration efficiency. The criterion considers not only the difference in quality of energy, but also the anergy＇s effect in heating. The physical concept of this criterion is clear, and the calculation is simple. It is preferable to any other existing thermodynamic performance criterion for, in addition to assessing cogeneration systems, it determines also the cost allocation of cogeneration systems.