The purpose of this study was to analyze the factors that influence the implementation of formative teacher evaluation. The subject of the study was a sizable elementary school in Taipei, which had participated in the two-year Formative Teacher Evaluation Project funded by Ministry of Education. To achieve the above purpose, this study adopted a qualitative approach and the data had been collected by observing, interviewing and analyzing documents. The results of this study were concluded as follows: 1. The implementation of formative teacher evaluation was influenced by three major factors, namely, instructional leadership, teachers’ culture and external influence. 2. The facets of instructional leadership that positively influenced teacher evaluation included promoting the professional development of teachers, establishing school-based evaluation and reducing teacher’s anxiety. However, inadequate communication impeded the practice of teacher evaluation. 3. The facets of teachers’ culture that positively influenced teacher evaluation include teachers’ commitment to their own professional growth, teachers’ inclination to share and coordinate. However, some teachers were hesitant about participating in teacher evaluation. 4. The facets of external influence that positively influenced teacher evaluation include the society’s expectations and support from authorities in charge. However, such support does not suffice to encourage teachers to participate in the evaluation: It does not help to reduce their workload, nor does it relieve them of their time pressure. Accordingly, this research offers some suggestions to authorities in charge, the elementary school, and the school teachers, which may also serve as a reference for implementing formative teacher evaluation programs.