This qualitative study, conducted in Hokkaido, Japan, concerns the investigation of the teacher cognition and practice of English as a Foreign Language (EFL) assessment and the impact of a professional development (PD) program on the participants. The PD program was carried out as a series of seven in-service workshops with five native speakers of English teaching in Japanese schools and universities. The workshops aimed to provide these teachers with a basic theoretical and practical understanding of performance-based language assessment. Another major purpose of the PD was for the teachers to conduct research and publish a paper concerning language assessment. In the study, the recording of teacher reflections was used as the data collection method. From the analysis of the data, the findings show that the PD program has positively affected the teachers. They have learned about alternative ways to assess the students, gained greater self-reliance, and became confident as teacher-assessors.