The authors evaluated usability of digital libraries and selected BTH digital library system as a case study. As BTH offers a lot of e-resources but users still feel difficulty in accessing these e-resources and services. The goal of this research study is to explain the importance of usability in digital libraries and as well to measure user satisfaction level regarding usability of BTH digital library system. These problems are identified by using different usability evaluation techniques with different criteria and possible suggestions for improvements have been given in this research work. Usability testing (Think aloud protocol) and usability inquiry (questionnaires, interviews) methods were used in usability evaluation of BTH digital library system. Usability evaluation has been done by selection of different usability evaluation criteria namely Satisfaction, Memorability, Learnability, Efficiency, Effectiveness, Aesthetic, Reliability, Simplicity, Visibility, User Friendly, Relevancy and Easiness.