One of the digital collections provided by university libraries is the Institutional Repository (IR). Library of UIN Maulana MalikIbrahim Malang provides IR to satisfy the information needs of its users. However, in its application, there are still problems experienced by users. Therefore, it is necessary to test the IR usability of the Library of UIN Maualana Malik Ibrahim Malang with the System Usability Scale (SUS) and discovery prototyping method. The purpose of this study is to discover the results of the IR usability testing at the Library of UIN Maulana Malik Ibrahim Malang and to find out whether SUS and discovery prototyping have a positive or negative effect on usability. The research method used is descriptive quantitative method, with the number of respondents as many as 100 students. The sampling technique used is purposive sampling. Data collection techniques used in this study are questionnaires, observations, and interviews. Data analysis used in this study is a formula based on the SUS method. Based on the results of data analysis, it is known that the usability score obtained is 50,1. This is due to several problems, such as basic search that does not work properly, improper navigation layout, and an unattractive interface. After the repairs were made, the score obtained increased by 28,725 to 78,825. This can prove that the prototyping is successful and the SUS and discovery prototyping method have a positive effect on usability.