Writing in L2 is considered a very challenging task by both teachers and students. Language teachers play a critical role in facilitating students' L2 writing development. Therefore, it is important to explore their beliefs about writing instruction as their beliefs exert influence on classroom practices. This study draws on the Repertory Grid Technique (RGT) to explore three teacher's beliefs about EFL writing instruction in senior high school in Taiwan. RGT, a qualitative inquiry evolving from Personal Construct Theory, involved three phases of data collection; firstly, the participant described activities in her writing class, which were later grouped and labeled. The activities were the elements, and the labels for activity groups were the constructs, which together formed a matrix for the participant to rate later. The rated matrix then underwent Exploratory Factor Analysis (EFA). The participant's beliefs were extracted as a result of EFA and follow-up in-depth interviews; sources of their beliefs were identified from analysis of the interviews. The results yielded eight beliefs related to teacher involvement, reading-writing connection, structural knowledge, student efforts, students' background knowledge, error awareness, status of EFL writing, and writing as thinking. Five sources of beliefs were identified, including previous learning experience, personal writing experience, personal teaching experience, training from a master's program, and in-service training programs. Knowledge of teachers' beliefs gives the participants an opportunity to reflect on their own teaching; it also provides information for teacher training programs and in-service training programs on enhancing quality and efficacy of teachers' EFL writing instruction.