To (1) diagnose the tour guides trainees’ communicative ability by using the computer-mediated simulation tour guide trainee test (CMS-TG Test), (2) identify the strengths and weaknesses of the tour guide trainees, (3) find out the test taking strategies of the tour guide trainees, and (4) to study their reflections towards the CMS-TG test. The population in this study included 160 tour guide trainees from the Academic Offices of Silpakorn University. They had the Test of International Communication (TOEIC) scores. Using the randomly stratified technique, the 30 sample tour guide trainees were classified into 3 groups: high, mid and low based on their TOEIC scores. There were ten in each group. The research instruments included (1) the Computer-Mediated Simulation Tour Guide Trainee Test, (2) the TOEIC Test, (3) Verbal Protocols, and (4) Structured Interviews. This study employed both qualitative and quantitative research methods. The research findings showed that the test could diagnose the tour guide trainees’ communicative abilities into three groups: the high, mid and low groups obtaining 94%, 71.09%, and 51.91% respectively. The strengths of the high group were shown in their language competencies and content knowledge while the mid and low groups shared obvious weaknesses in language and communication abilities. Moreover, the three different ability groups employed different test taking strategies. Finally, the CMS-TG Test was perceived as a beneficial instrument in assessing tour guide trainees’ communicative ability.