This study aims at exploring the level of Thai EFL undergraduate students' willingness to communicate in English language online classrooms. The crucial evidence from students' voices was investigated through the mixed-methods paradigm, called exploratory sequential research design which employed a qualitative, followed by a quantitative approach to answer the main research question that investigated the levels of Thai EFL undergraduate students' willingness to communicate in English language online classrooms. In the first phase, qualitative data was collected and analyzed from focus group interviews to elicit the rich data in deep detail and explain some outstanding points of view from the participants at the university level. Then, the results from the qualitative phase were used as a guideline to develop the questionnaires which were relevant to the willingness to communicate in online classrooms. One thousand one hundred and nineteen participants (N=1,119) responded to the questionnaires. Results showed that the salient factors that influenced the level of students' willingness to communicate in online classrooms were communication channels, students' beliefs, peer influence, teachers' characteristics, teaching methods, and online classroom atmosphere. The findings of this study can create teachers' awareness when teaching online and can be used to seek out how to improve themselves and adjust their own teaching approach to enhance their students' online language learning outcomes in the future. Recommendations for further research are addressed in the end.