This study aimed to explore the EFL teacher's reflective practice in the implementation of Audio-lingual Method, investigate his challenges and explain his technique to improve students' vocabulary mastery. The case study was carried out in SMA Pasundan Cikalongkulon Cianjur. The respondents were 32 students and an English teacher who implemented the Audio-lingual Method. They were chosen purposively to be involved in this study. The instruments of data collection were carried out by observing the teaching-learning process of the Audio-lingual Method and interviewing the subject teacher about his technique and challenges. Based on the result of the observation and reflective practice showed that the EFL teacher's implemented Audio-lingual Method well. It was proved that he applied its principles namely imitation, using the target language communicatively, separating the native and the target language linguistic system, modeling, repetition, reinforcement, verbal and nonverbal stimuli, and English culture. Moreover, the student's average score improved, and it obtained 94. There were some challenges to improve their vocabulary mastery namely limited time, student motivation, different capabilities, and the classroom condition. Moreover, there were some techniques used by the teacher to improve students' vocabulary mastery such as dialog memorization, repetition drill, transformation drill, and question and answer drill, and the most applied techniques were repetition drill, transfonnation drill, and question and answer drill. It is recommended that EFL teacher's reflective practice can be implemented in the other methods, skills and aspects in order to get meaningful teaching learning process. Furthermore, study could be conducted to investigate how the students feel when learning vocabulary using the Audio-lingual Method.