Speaking English in a face-to-face classroom versus an online classroom may make EFL students feel uneasy in different ways, depending on a variety of factors. To help students reduce anxiety, teachers must understand the different levels of anxiety they experience in different situations. This study investigated the differences in English-speaking anxiety between face-to-face and an online classroom for Thai students, and used questionnaires and structured interviews to identify the factors that may contribute to the anxiety of different students. The questionnaire was completed by 26 first-year undergraduates from a Thai public university who had both face-to-face and online classroom experience in order to compare anxiety levels in the two learning environments. Seven students were then interviewed in order to determine the factors that may have contributed to these differences. The data from the questionnaire were presented as mean scores and standard deviation (SD) in order to compare the differences in students' anxiety between on-site and online classrooms, whereas the data from the interviews were used to explain the factors that could cause these differences. The results of the questionnaire revealed that, with the exception of confidence, students' speaking anxiety in face-to-face classrooms is greater in every respect than in online classrooms. In the meantime, the results of the interviews revealed that nervousness, a limited vocabulary, and an inability to remember vocabulary could be significant contributors to anxiety differences. The discussion explained how each aspect and factor makes speaking anxiety in on-site classrooms greater than in online classrooms, and suggested and encouraged teachers to reduce students' anxiety in both types of classrooms. Teachers can improve the language learning outcomes of their students by creating a less stressful environment in the English classroom and by incorporating activities that motivate or engage students to practice speaking.