The purpose of the study was to develop an instructional model to enhance student teachers’ ability to design critical reflective instructional lesson plans and to apply the designed instruction into classrooms. The participants were 15 student teachers in Rajamangala University of Technology Thanyaburi in Thailand. There were 5 steps in the instructional model: 1. Technical Reasoning step (T), in which the participants were encouraged to examine and identify the selected teaching method. 2. Reflectivity step (R), in which the participants reflected the weakness and strong points of the selected teaching method. 3. Methodology Improvement step (I), in which the participants were to improve the weaknesses of the selected teaching method, and design the new teaching method. 4. Instructional Application step (A), in which the participants were to apply the newly designed teaching method in actual classrooms. 5. Critical Reflective Thinking step (C), After applying the TRIAC Model to 15 student teachers, it was found that their ability in designing instructional lesson plans were done in more critical reflective process than before participating in the Model. In which the participants were to exchange their data and experiences, evaluating the effects of the designed instructional methods among their colleagues and concerned public.