This study examined how well Thai teachers of English as a Foreign Language (EFL) perceived their teacher preparation institutions trained them. The researcher focused on in-service EFL teachers who graduated from formal teacher training institutions in Thailand and were within their first three years of teaching in a high school or primary school setting. A survey was designed to diagnose the source of under-prepared EFL. SPSS was used to analyze data. Based on descriptive statistics data, the findings indicated teacher demographics, education background and current work setting influenced their perception of preparedness. EFL teachers who felt prepared, majored in English for Business Purposes, had a minor related to teaching, had a smaller class size, taken subject specific courses and completed training and one year of teaching. These findings may benefit higher education leaders who revise EFL teacher preparation programs, educators who develop professional seminars for new EFL teachers, and foreign educators who work with Thai students or educators.