Web-based learning systems (WBLSs) allow learners to choose any learning topics according to their own needs and progress. This may be the reason why the WBLS is a popular learning tool in educational settings. Thus, it is necessary to investigate how to help learners effectively obtain their desired results. To this end, the usability evaluation of the WBLSs becomes an essential issue. To address this issue, this study assesses the usability of a WBLS with real users' opinions and heuristic evaluation. Our findings show that the results from Nielson's heuristics and those from learners' questionnaire share some commonalities but there are also differences between them. Therefore, these two approaches should be used together so that a complete set of guidelines for the design of a WBLS can be developed.