User interface of digital libraries (DLs) has a critical role in optimal performance of these libraries. In fact, optimal performance of DLs depends highly upon the appropriate interaction between DLs and their end-users. In spite of the importance of user interface in DLs, few studies have been done in relation to evaluating DLs user interface, in general, and specially in regard with criteria evaluating them. On the other hand, most of related studies deal with "usability".In some of these studies, user interface is eonsidered as a sub-part of usability. As a result of variation of criteria posed in the literature and lack of special criteria for evaluating DLs user interface, the present study aims to identify as well as to determine such criteria via examining related texts and resources comprehensively. Using documentary method and content analysis, the study was performed.To do this, Web and some leading databases including ERIC, LISA, SagePub, Proquest, Emerald, ACM, and ScienceDirect have been searched. Thus, after scanning evaluation-oriented resources and other resources whether theoretical and empirical, 43 resources were categorized. Based on the contents of these resources, 3 sections “user interface and DLs”, “DLs and usability”, and “other studies related to user interface” were identified. Finally, on the basis of occurrence of relevant criteria available in the literature, evaluation criteria of DLs user interface have been determined and proposed for evaluating DLs user interface.