Introduction. This study compares 30 blind and visually impaired users’ assessment of accessibility and usability of the two mobile platforms (mobile app and mobile web) of a digital library.  Method. Triangulation of data collection was applied, including interviews, think-aloud protocols, transaction logs, post-platform interviews, and post-search interviews. Analysis. Two steps of analysis were used. First, quantitative analysis was applied to compare assessments of participants towards two mobile platforms’ accessibility and usability. Second, qualitative data were analysed to identify types of design factors. Results. Mobile app performs significantly better in all accessibility and usability variables except accessing information/objects. Most importantly, nine types of design factors are revealed in relation to blind and visually impaired users’ assessment of accessibility and usability for the two mobile platforms. Furthermore, the design problems of Mobile Web are associated with a responsive design that adjusts the digital library interface to a mobile device, the complexity of digital library structure and formats, and a sight-centred design that excludes blind and visually impaired users’ unique information-seeking behaviours.  Conclusion. Mobile platforms of digital libraries, especially mobile web, need to improve their designs. Design implications for  mobile web are further discussed.