Willingness to Communicate (WTC) is among the concepts that has received considerable attention in EFL/ESL research in recent years as it brings together a myriad of factors that explain why L2 learners do or do not engage in L2 communication. An EFL classroom provides an environment that promotes use of the target language, yet Thai learners are often reluctant to communicate in English in the classroom. The present study aimed to investigate the possible factors that influence Thai learners' WTC in the EFL classroom context. Participants included Thai students in both the Thai and International Engineering programs at a public university in Bangkok, Thailand, as well as both Thai and foreign instructors. The assumption underlying the investigation was that WTC in English in the classroom of Thai students in both Thai and international programs was is influenced by variables other than their English proficiency. Likert-type WTC questionnaires and semi-structured interviews were used to collect the data, and a quantitative descriptive approach was employed. Content analysis was also used to analyze qualitative data and provide more in-depth answers to the research questions. Statistically significant differences in WTC between Thai EFL learners in the Thai and international programs were found both in degrees of willingness and reported factors influencing the WTC. The results have been used to develop evidence-based guidelines for English instructors in the implementation of classroom activities and teaching methods to promote Thai learners' WTC in the classroom.