Lesson planning employs both pedagogical and content knowledge. The processes are complex and student teachers, undergoing practicum, struggle to plan. However, studies that explain processes drawn from a qualitative inquiry to explain the practice are rare. The purpose of this study is to generate a theory to explain Thai EFL student teachers' lesson planning by adopting the grounded theory. The research questions include 1) How do the Thai EFL student teachers design their lesson plans? 2) How do the student teachers implement the lesson plans? and 3) What action do the student teachers take after implementing the lesson plans? The research instruments are semi-structured interviews as well as observations of 22 student teachers majoring in Teaching English. The data is analyzed by means of coding to identify emerging categories and generate a substantive theory. A constant comparative analysis of the data generates a grounded theory of EFL student teachers' lesson planning, illustrating cyclical processes of four stages. The first stage is pre-planning, where personnel and institutions have an influence by giving information necessary for planning. The second stage is planning, showing both linear and non-linear processes. The third stage is implementing plans, observed by school and university supervisors. Student teachers agree, partly disagree or entirely disagree with feedback and use or do not use the feedback to improve subsequent plans. Finally, the last stage is reflecting/evaluating, showing modes of communication and a reflective process for both problems and success.