Usability testing is a method that determines how userfriendly a website or a mobile application is. The present study investigated the usability of a newly launched mobile application called TU-GET CBT or Thammasat University General English Test (Computer-Based Test). It also strived to understand the extent to which the application contributes to users' motivation. Twenty-one target users were involved in three focus group sessions; they also completed a usability and intrinsic motivation questionnaire. In addition, five experts in the field of language education and technology performed the heuristic evaluation that measured the application's usability. The findings showed that in general, all participants were satisfied with the application in terms of its usefulness for English language learning and test preparation. Nonetheless, several technical usability issues were identified including accessibility, navigability, and aesthetics. Pedagogical usability of consistency and feedback were also raised across findings. In regard to motivation, the findings suggested that the users' motivation can increase if they perceive the application as valuable and enjoyable. These findings provide valuable implications for the next phase of the TU-GET CBT application development.