Purpose This study aims to evaluate the usability and information architecture of the digital library (DL) website of the National Library and Archives of Iran (NLAI). Design/methodology/approach This applied study used an exploratory mixed method, card sorting. Data were collected by interviewing, observation, usability test and card sorting. By interviewing users about problems of the DL, eight tasks were predefined and users’ problems in the path were identified. Their satisfaction of the tasks and the usability rates were measured via a questionnaire. Card sorting was done to inform on the information organization of the website elements. The study population included all users of the DL of the NLAI in two groups: ten initiator users (public users) and ten expert users (librarians). SPSS was used for analysing the usability test quantitative data. MaxQda was applied for analysing interview-driven qualitative data. Qualitative content analysis, categorization (data organization and grouping) and determining main and secondary codes were applied as well. The sort optimal application was used for analysing card sorting data in the form of similarity matrix and dendrogram. For validating qualitative findings, triangulation was used. The internal reliability of the used questionnaire amounted to a = 0.87 Findings Regarding the assigned tasks, new initiator users by consuming 367.67 s for registering and new expert users by consuming 403 s spent the most time. Task 2 ranked first in being incomplete among 40% of initiator users and Task 3 ranked first by 30% of incompleteness. Expert users had more unsuccessful attempts. Task 5 with the mean rate of 3.35 and Task 8 with the mean rate of 2.25 were the most difficult and the easiest tasks, respectively. Some usability components were rated lower than the moderate point. Only 30% of initiator users and 10% of expert users were satisfied with the website. A total of 12 categories and 452 codes were identified as main problems of the DL. The problems related to a vague perception of concepts and labels with 90 repetitions and digital source display with six repetitions ranked as the first and the last problems in working with the DL, respectively. The sort optimal package produced the card sorting results as a matrix similarity and a dendrogram. Card sorting reflected some changes in organizing information items. Interviews after card sorting emerged some new groups to be included, such as links to other digital libraries, shared databases in the organization and frequently asked questions. Originality/value The library’s website should be designed in a manner so that it can satisfy users with different traits. As the information technologies are increasingly developing, the importance of such a design increases for better service provision and effective competition.