The rapid evolution of Information Technologies (IT), especially with the coming of the Internet, has existed online library catalogues which have a vital part to play in Educational life. The online library is a type of information retrieval system. It is a library in which collections are stored in digital formats. These formats may be stored locally or entered remotely via computer networks. To assess the success of online library catalogues; usability criteria should be considered to ensure that users can find their way quickly and easily to e-resources. This paper aims to use thinking aloud protocol to evaluate two distinct interfaces of library catalogues to examine how their usability problems significantly impede user performance. The results show that the usability is the main factor affecting users' performance, and it is the best attribute to measure the extent to which the online library catalogues meets the requirements of their users. © 2014 IEEE.